If a credit report has been done & you have given the lender documentation needed then a pre-approval (or credit approval) letter should mean something. A credit report is good for 90 days for Conventional loans & 120 days for a Government loan. As long as there are no derogatory changes...assets, income, debts...the above info should tell you how long the pre-approval is good. The appraisal or problems with title could change that. Just be sure you have a reputable lender & you should be fine. http://answers.yahoo.com/question/index?qid=20110602130454AAsidIp
Usually the home loan prequalification is good for about 120 days but it also depends on the policy of the financial institution from which you are going to take loan. You must up date your prequalifications on regular basis so that you can get loan at ay time from any where.
Usually a loan prequalification is good for 90 days. If you go beyond the 90 days, the lender will usually extend it. Double check you loan prequalification for the exact good thru date of your letter.