I've never had to return to port because of fuel reasons, I'm very careful with my diesel...
I usually start heading for home when I have 1 or 2 eels left, 1 aft and 1 forward, or just the 1 aft. This is because I tend to hang around in places like am36, the Irish sea and other traffic-heavy shallow water areas.
In those types of patrols I've never not met something juicy on the way back, so it's nice to be able to sink it.
Nothing is quite so frustrating as seeing HMS Hood limping into Loch Ewe at 5 knots because I smacked her up good with three eels a few days earlier, but didn't quite take her to the bottom, having to give up the chase because of foul weather.
And then having spent my last eel two hours ealier on a tug of all things....
Same goes for those long long patrols (50 days or more) to the Carribean, you want to have at least one eel to take out any interfering DDs in Biscay hampering your triumphant return..
