Kerryann is correct; most are good for 90 days. Some things to remember while you are shopping, don't do anything to jeopardize the pre-approval. Pay your bills on time; don't make any major purchases on credit without telling your lender first. I had a client purchase a car once and it almost blew the deal up.
When you finally find a house and make an offer you will need to update your loan file with fresh pay stubs, bank statement and any other documentation your lender requests. Keep track of these things so you can have them available.