When a delay occurs with finance approval, your solicitor / conveyancer will request an extension from the vendor. Usually it is granted as the vendor will not want to lose the sale but depends on the time period required. Recently I sold a property with a 6 month settlement - the chinese group continually requested extensions - at one point - the solicitor and me as vendor ran out of patience and told them to go jump - the contract was rescinded but now I can find a new buyer and I believe there will be a higher price.
As a general rule - always sign subject to finance - so many don't.
Cheers, Ivan