It depends on:
1. The Vendor and their plans and ability or otherwise to vacate. Is it an executor's sale? Are there any problems with the house and the title?
2. The Vendor's solicitor and see above
3. The Vendor's bank
4. You as a purchaser - are you in funds ? Have you a formal letter of loan offer?
5. Your solicitor - and see above as to who they need to do their bit.
6. The property - have you had it surveyed?
A realistic time frame - with everyone willing and no issues is 8 weeks. It is possible to do a transaction in 1-2 weeks but only if everyone is keen and everything is in order - which almost never happens.
The big delays are usually with the purchaser organising their finance and getting life cover and then the loan cheque.
mf