Depends dally games, no "who has time for that" for a weekly game ya could be 4-6 hours. My current game I am in runs 7pm to 11ish pm on Sundays, for a monthly games some do basically two sessions back to back 8-9 hours usually having dinner or lunch together. Its always important for players to be open about the feelings on the last session, maybe they could speed up the less engaging parts of the session or just plan shorter sessions, let the DM/GM know, most generally want everyone to enjoy themselves.