First, understand that this is not a legal opinion. Talk to your attorney. A contract is usually considered to be complete when your attorney has completed a review of the contact and signs off on it. But some cautions are in order.
If you are using a Realtor, he/she prepares an initial draft to make the offer, and when it is signed by buyer and seller, it creates an "offer and acceptance" which is then given to the Attorneys for their review. You have three days to seek an attorney, or you forfeit the right to an attorney review. The clock starts when both parties receive completed paperwork, properly signed. Attorneys can extend the initial three-day review period, and frequently do. Attorney review takes as long as it takes.
If you are not using a Realtor, your Attorney prepares the contract, and it becomes official when both Attorneys (buyer's and seller's) sign off on the contract. You in effect skip the "offer and acceptance" stage.