How long were they normally submerged in the patrol area?

These submarines wanted to be on the surface every night for multiple reasons if that was possible. You might come up to the surface at dusk and dive again at dawn. If you were in the tropics, the day/night split would have been close to 12 hours each. However, if you were in higher or lower latitudes, the split could be much less equal and you might have been submerged a few hours longer in summer. That leaves less time to recharge the battery.