I'm not sure what you mean by "prequalification" If you have made an actual full loan application, your rate quote and approval are good for a period of 60-90 days--ask the bank who gave you the approval. If it was a "quick" prequal over the phone without a full credit check (which will cost you money), there are no guarantees as things change daily. No rates locked in--it is a "guide" for what you might be able to borrow to buy a property. When dealing with a bank in general, the only "guarantee" is what they tell you today, and you lock it into an actual agreement. Finances are fluid and change depending on what you buy, charge, finance at any given time. Speak with your bank regarding the particulars of their requirements and "guarantees".
I hope this helps,