It can take weeks for an offer to be ratified. Buyers and sellers often counter back and forth for weeks before reaching mutual agreement on both price and terms. In this case, it's a good idea to re-evaluate the closing date in the contract before inking the final agreement.
Some buyers make offers that propose closing a certain number of days from acceptance of the contract, often 30 days. Even if you negotiate for a month, you will have 30 days, or whatever number of days agreed to in the contract, to arrange financing, complete inspections and close the transaction.
However, sometimes closing is to occur on a specific date, say June 1. If you start negotiating on May 1 and it takes a couple of weeks to arrive at agreement, you may not be able to clos