When your laptop stands by, it could do this in two different ways: sleep and hibernate. During sleep computer continues to consume energy. That’s because it simply turns off most power-consuming nodes like monitor, CPU, etc., but other chips are still powered on. Thus laptop could sleep until it drains battery, usually it could take several hours or days.
Hibernate is a different approach. When the laptop is going to hibernate, it save current state of CPU, operating memory and other devices to system hard drive and powers off. This operation could take noticeable time. As long as hard drive do not need any energy to save it’s state, your laptop could be hibernated for years, and after that restore it’s state exactly like it was at the moment of hibernation.