You make the decision about returning to base. I always advance to my grid at maximum speed of 'ahead one third' and then patrol my grid area at 'ahead slow' in the IIA to economise on fuel. Try to guesstimate how much fuel you used getting to your patrol grid from the gauge and make sure you have at least that amount left when you decide to head for home.
Do not rely on your navigator's fuel reports! He just shouts out the first number that comes into his head when you ask him about remaining range.
Good luck, Herr Kaleun, and the band will be waiting for you!