Normally 3 to 6 months, the reason is because the traditional mode of communication, i.e letter and the buyers' financier (assuming you take a loan from bank) to conduct a valuation on property and so on.
But it does not mean you can't do anything. You can help to expedite the process by:
1. both seller and buyer using the same solicitor;
2. instruct the solicitor to revert documents by uploaded to email rather than ordinary post;
3. provide adequate documents to banker (if any) to secure a fast letter of offer.
4. (for investor only) negotiate with seller to allow entry to premises before trasfer completed, talk to your lawyer for this, so you can let the premises for rent.