How Long should can you play while making sure your game stays strong?

When I did end up losing money, I would always notice it was never within the first hour, rarely the second, and most definitely sometime after that.

How long can/should someone play before they stop playing?

It depends on the game and the person. It also depends on the training and the bankroll. It could be that after 2 or 3 hours the sharks have figured you out.

today i played for a solid 8 hours... and my first 4 hours i final tabled a tourney

This last 4 hours god... im making terrible plays

today is my first day of getting back into online poker though so maybe im just not used to it