Your question doesn't make sense. Do you mean how quickly you must sign it? In which case it's between the two parties in question. Have you been offered a job/given a new contract that you are not happy with or something?
I wouldn't have done that tbh. When I issue employment contracts for new starters I always put a date they must have accepted the offer by - you don't want the situation where your new employee decides to take up the offer a few months down the line by which time you've given up and employed someone else.
Agree with flowery but you can send them a further letter saying that you need a response by x even after doing this and resending the contract. Your original offer would then be cancelled by the counter-offer.