You can start shopping for a mortgage at any time (you probably should have already started). Once you decide on a bank and start the process figure 45 to 60 days to get a mortgage approval and close (you can get pre-qualified, but this is only a preliminary step - there will be more processing). Once you start the process, you can lock the rate in at any time. Usually rates are good for 30 to 60 days, but each bank/lender has different policies, so make sure you ask about the locking (some banks, for example, will lock in for 30 days and charge a fee to lock in for 60 days; others will give you a full 60day lock in).

You should apply to get pre-approved for a mortgage right away. Your contract, if it is subject to financing, typically requires you to apply within a certain time frame. Pre-approval can usually be done almost immediately if you have your documents (tax returns, pay stubs, W-2’s, etc.) available. Typically you can lock your rate once you are approved, if you feel the rate offered is attractive. Be aware that locking in a rate for over 30 days (which you may need to do to allow sufficient time for the owner’s lender to respond) may cost you. Consequently, you may want to consider floating the rate until you get the lender’s approval.