Chess Stamina

Playing Blitz online, I've noticed that my day generally starts with a +50-100 pt gain above my daily average. However, after several hours, and a few dozen games, I start to slump quite a bit, often falling around 50 or so points below my daily average. It's becoming a daily cycle of starting my day recovering lost rating points from they previous afternoon/evening, then eventually backsliding again. Mental fatigue is expected, but I was wondering where others were with this. How much Blitz can you play throughout the day (or in one sitting)?

Take a tip from the gaming community - quit while you're ahead.

Thanks for the feedback. Agreed, I play too much blitz, but that's not what I was asking.  How many games (or how long) are you able to play, at your highest level, before you start to flounder?