When to stop your workout?

[Sprocket]

I completed a nice workout last night, pushing myself a bit. For the first time in a very long time (used to lift back in high school) my arms stiffened up and I had a nice burn in my chest and abs. My breathing seems to be catching up too.

When do you call it quits? Until you just can't do another rep or what?

 

[tic]

When that HOT chick leaves and motivation is gone. The blood stop pumping.

For real though, when I am completely exhausted after the muscle group that I workout is pumped and fatigue. For example, on chest day, when a normal push hurts like crazy and struggle to do and sometimes impossible to do.