You might have more losing sessions, but this doesn't influence your overall winrate unless you're developing great reads and that's making the difference, or maybe you play better at the end of a session.

However, I wouldn't limit your session to a certain time, especially if you find a great table. I would quit playing when I feel like I'm not playing my best game (tired, upset), not enjoying myself, or the game isn't worth it. Of course, there's always another game...