A night has as many parts of the night as there are full 3-hour blocks in it. This means that at night of 9 hours 44 minutes has 3 parts, whereas a night of 7 hours 3 minutes has 2.
The part of the night concept is mostly relevant for Hunting and Training, and offers some guidelines as to what a character can do in that night. It is not meant as a strict division of the night into parts, where every activity takes up one or more parts — in a night of 7 hours it is still possible to hunt for 1 part, train for 1 part and visit Elysium for about an hour.