A decision in principle (DIP), also known as an agreement in principle (AIP), typically lasts between 30 to 90 days, depending on the lender. The DIP is an indication from a lender that they may lend a certain amount based on preliminary information you provide about your income and outgoings. It's important to note that a DIP is not a guarantee of a mortgage offer and is subject to a full mortgage application and credit check. The duration of the DIP is crucial for timing your property search and purchase, as you'll need to find a property and make a formal mortgage application before it expires.