There is no fixed time - its a matter for the parties - or more likely for the vendor.
When contracts issue, that generally follows a period of time when the house has been viewed, negotiations have taken place, vendor preumably has instructed solicitor to take up deeds and purchaser has been busy organising finance ( at least in general terms). As there is no binding agreement in place until contracts are signed, exchanged and a full ( or agreed lesser deposit) has been paid, both parties ( and certainly the vendor ) should (?) want contracts to be binding. Of course purchasers may have been entirely economical with what they told estate agent and may be weeks off arranging their finance so that ( if they were prudent) they should not be signing contracts. Contrary to the oft expressed view that the solicitors should not let their clients sign contracts without making sure they have loan approval in place, that is a decision for the purchasers to make. The solicitor can only advise - the clients make their own decisions. A lot of people entirely underestimate the rigours of house purchase and can be told nothing.
It would be the norm for the deal to go from contract issue to complete sale in approx. 8 weeks but each set of circumstances is different.
mf