Tricky question.
I drink two cups a day when I wake up. If I have more than 2 or any at all after about 10 am I'm awake all night instead of only half the night.

I think it's one of those things where it depends how ones health is to begin with. Some people probably can do without any caffeine at all. Other people it doesn't seem to harm them. If I drink too much caffeine, I am more prone to anxiety attacks and more RLS symptoms and I get a bit ticcy. It helps me organize my thoughts better though.

Are you just generally asking or are you wanting specific information for specific health conditions?