The best thing to do is speak to a loan officer to make sure that you are shopping in the right ballpark before you have a realtor take you out, and make sure that your income, assets and credit qualify you for a loan.
I'm not sure which banks you are referring to. The mortgage companies I work with generally issue preapprovals that are valid for 3-6 months. It is best to speak with a loan officer to make sure you can purchase before you go out to view homes. That way you know your actual price range before you start looking and fall in love with a home you can't actually afford.
Call me so we can get you on your way to home ownership. 302-355-1865.