How long could a U boat stay underwater?

German U-boats from World War II could typically stay submerged for about 24 to 48 hours, depending on the model and conditions. Their batteries would allow for limited underwater operation, usually around 12 to 24 hours at cruising speed. However, they would often need to surface to recharge their batteries and replenish air. Advanced models and techniques, such as schnorcheling, extended their underwater endurance somewhat.