Who does what and when
How long it will take from an offer for  propertybeing scceptd, until the moving day, depends on a number of factors.   The average process takes between 8 and 12 weeks.
The first thing to bear in mind, is that every transaction is differnt.
Different property, different people, with different needs and expectations.
It can be quicker or slower, depending on whether or not there is a chain of trnsactions.  It usually takes a little longer for them all to come together, ready to exchange contracts and we can only go as quickly as the slowest party in the chain, although we will do all we can to speed matters up.
Problems with the title to the property which need to be resolved can cause dleay.  As can problems obtaining documents which are required.
If there is a delay in the issue of a mortgage offer, this can als slow up the process. 
Buying a leasehold flat , normally takes much longer than the average transaction, normally due to dleay in the Seller and the l=flat management company providing the requisite documentation for consideration.
If a lease needs to be extended, that can also slow up the process, sometimes, by months.
The speed of purchase of a property in the course of construction is dependant upon the construction of the proeprty being completed and the relevant building regulation completion certificate being provided.  For example, bad weather can delay the completion of a proeprty.
Even transaction involving 2 proeprties in the same street, can raise different legal issues.  One may be registered land, and the other not, requiring different legal processing.  One may have been extended and altered, requiring checking of planning permissions and building regulation approvals and the other not, and so on.
Basically speaking, the process can be divided into two.
Firstly, the pre-contract stage, when all the investigations are made to ensure it is safe for the property to be bought, and which generally takes  the longest - about 6 to 8 weeks.
Secondly, the post exchange stage, up to completion, when you can move in, followed by registration of title at the Land Registry.
The steps in the process, are as follows:-
1.  We take details from you, of the terms agreed and establish who the solicitors for the other party are.
2.  We obtain documents to verify your identity, nd address and if you are buying, the source of your funds.  If you are selling, we will need details of your lender, to verify the sum to be paid, to pay off any mortgage on the property.
3.  The Seller's solicitor will obtain details of the title to the property, prepare the draft contract, arrange for the Seller to complte a Property Information Form (giving details, such as who owns the various boundaries if any work has been done to the property and so o), and a Fittings and Contents Form (detailing items at the proeprty, included or excluded from the sale).  The Seller's solicitor alos, hopefully, btain copies of any relevant documents, such as planning permissions, building regulation documents and guarantees.  The Seller's Solicitor then sends all this documentation to the Buyer's solicitors.
4.  The Buyer's solicitor considers the pack of papers, and raises ay queries with the Seller's solicitor.  The Buyer's solicitor, will also at this stage, put in hand the appropriate searches for the proeprty.  These will be te local search, with the local authority, the drainage and water search with the local water company and an environmental search.  The local search will show if the local authority have any records of any matters which could affect the property, such as proposals for a new road which would run through the property, and if the road outside the proeprty is maintainable at the public expense, and what planning and building regulation approvals there are  for the proeprty and if any notices have been registered againt the proeprty by the local authority.  The drainage and water search will show if the proeprty has mains water and sewerage, the location of the neaest mains pipes and sewers and if any of these run through the proeprty, as well as infrmation such as the location of the water stop cock.  The environmental search will reveal any environmental problems that might require investigation, and in particular, if the proeprty is in an area which suffers from flooding.  Any queries arising from the search results may may need to be raised with the appropriate authorities, the Seler's solcitors and or yur surveyor.  There are also furtehr specialist searches which may eed to be carried out, depending on the location of the proeprty.  For example, a mining search if the proeprty is in an area wheere there has been mining.  The Seller's solicitors will investigate ny queries raised and take instructionsfrom the Selleras to teh replies to be made and reply appropriatley to the Buyer's solicitors.  They will then consider the replies and if not happy with them, will go back to the Seller's solicitor on the relevant points, until they are satisfactorily resolved.
Time for some conveyancer's jokes about pre contract enquireis.
Have you heard the one about a london solicitor asking a Welsh solicitor for the name and address of the suplier of radon gas?  Answer - "supplier God, address heaven".
Or what about the query "is the proeprty haunted?"  Answer "yes, a very friebdly ghost called Gerald - he is really part of the family, but sadly cannot move with us."
5.  It is also, during this period, that you will arrange to have a survey carried out on the proeprty, if you are buying and let us know, if it brings up any ppints which weneed to query with the Seller's solicitors.
6.  It is also in this period, that the Buyer will make an application for a mortgage ad the Buyer's solicitors will receive mortgage instructions from the lender to act for themand explain any relevant mortgage conditions to the Buyer and raise any matters that need to be referred to the lender for its consent to proceed.
7.  When all the above wot=rk has been satisfactorily completed, then exchange of contracts takes place.  The Buyer and the Seller each sign their part of the contract and the Buyer pays a 10% deposit part of the purchase price.  A compltion date is ageed (this can often be a fraugt subject, especially if there is a chain and a number of people all have to agree on the same date) and contracts are exchanged by the solicitord and all partiesa re bound to go ahead.
8.  In the period between exchange of contracts and completion, the Buyer's solicitors will send requisitions on title (mainly queres thees days on the mechanics of completion), such a =s whre th kets can be collected from at completion and teh bank detials of the Seller's solicitors for the transmission of the monies on the completion date),  and draft and send the draft transder deed to the Seller's olicitors for approval.  They willthen reply.  Increasingly however, this process is done before exchang of contracts, to try to shorten the time between exchange and copletion.
9.  The Seller's solicitor will get the Seller to sign the transfeer and cotact the lender for the figure due to pay off the mortgage on the proeprty and the esatte agent fordetials of their fees.
10.  The Buyer's solicitor will get the Buyer to sign the transfer deed, the mortgage deed and the approval of the Land Transaction Retturn Form in Wales of the Stamp Duty Land Tax for in England that the solicitor will have prepared.  The Buyer's solicitor will also carry out final searchs at the Land Registrym check the results, prepare and submit a report on title to the lender and request and process the mortgage monies as well as get n any balace that has been calculated as due to complete by the solicitor from th Buyer.