Good morning,
I have noticed I have been deteriorating after about 2k-3k zoom hands, which answers my own question of when I should be taking a break; but what do you guys do? Do you plan your session out before it starts or do you just play until you feel you are slipping?
Flooooky
Taking breaks every hour works for me. Not long breaks, something like 5-10 minutes. Look at some hands played, analyze them, post them somewhere, ship some expert on skype 
and then back to the game. 
Hey
I think it just depends on how you work really. I tend to take breaks depending on how the session is going and how I'm feeling. If I feel like I'm about to tilt then I try to quit, but that doesn't always work 
Also if I make some stupid overambitious spew then I start questioning whether I should continue and consider stopping the session. And on top of that I try to have my stoploss of ~3BI or so.
If you are more regimented with your game in that you play for a solid period of time then I'd say around the 45-50 minute mark would be a good time to take a short break of 10-15 mins. Get up, walk around, grab a snack etc.
Really depends on your style I think, everyone is different. Sounds like you've figured out the point where you need to take a break which is good.
Lewis
I do play 1 hour, take 10 mins break, then post at least one hand, take notes of marked hands on oponents, then take 5 min break and then again play.
