"I've just about had it" = "I'm losing my patience!" or "I have little desire for it to continue" You will also hear, "I've just about had it with you" = I'm losing my patience with you" "I've just about had it with x" "I'm losing my patience with x".

I say this when students in my students keep on talking after I've asked them to listen a million times.

Yes. I am about to lose my patience. I can't take any more.

that means I can't stand it. 我简直受够了。