I don't seek to be a nit-picker or anything, but as I understand it,
chocolate has little or no caffeine. Instead, it contains a compound
that has a similar effect -- theobromine.
I have read that caffeine has a 12-hour half-life in leaving the system
after consumption; I have never heard comparable numbers for theobromine.
Reportedly theobromine is bad for cats; however, I met a couple some years
ago in Florida whose cat loved to eat Hershey's chocolate bars. They lived
on a lake near Orlando and had a dock in their backyard. They claimed
that their eccentric cat would sit on the dock waiting for fish to swim
by, jump in, and if the cat caught a fish, kitty would eat it. I never
had the opportunity to observe that behavior, but a cat that eats
chocolate is certainly unusual. :-)