Typically, the approval in principle mortgage lenders issue lasts between 60 and 90 days, and an online mortgage promise should be exactly the same. However, it’s important to think of the agreement as a ‘live-system’.here are a number of benefits to getting an agreement in principle before you find a property: 1  You know you have the money for the purchase in the first place. 2  It gives the vendors and agent confidence that your offer is credible and that obtaining the finance to by the property is viable.