The thing they will (should) appreciate the most, and the thing that will keep the bridge most intact after leaving, is to be generous in the advancement of your notice. This honors the trust they put in you as an employee with significant responsibilities by affording them the valuable time to absorb the impact to their company in a managed way. Since two weeks is customary, something on the order of 4 weeks would be in good faith of the trust between you.

Some might say by giving extra notice you risk "showing your hand" and being fired prematurely. However, if you have a good personal relationship, this is unlikely to happen.