Its how long is a piece of string , unfortunately!
In the ideal world, contracts would issue this week, purchasers would have their loan approval and their life cover all sorted within a week to ten days after that, the "legals" would be easy and everything could be tied up/wrapped up with 4 weeks.
The reality , in my experience, is that Vendors and Purchasers are still massively naive/overly optimistic about the rigours of getting loan approval, complying with the terms, extracting the loan cheque from the lender, etc.,etc.
The best thing to do is to set a realistic time frame of 8 /10 weeks to complete and ask that contracts be signed and exchanged within 2/3 weeks.
After that, cross your fingers, say your prayers and don't count on anything until the day the sale proceeds are banked.
mf