There are the length of playing the "whole" game, the length of a minimum/typical/maximum session, and the length of a supported unit of play, and each can be long, short or irrelevant.

For example, an online PvP game has discrete games (e.g. a worst case of 2n minutes to play chess with a time limit of n minutes), one often plays a few games in a row depending on fatigue and available time, and replayability is infinite.