I metabolize caffeine slowly and have practically no tolerance. A medium-sized latte anytime after 5 PM will keep me up all night. A decaf equivalent will make it really hard to fall asleep (remember that decaf still has caffeine).
I think my limit when not ingesting caffeine in the early morning is about 200mg before I have issues with falling asleep.
I've seen research suggesting the use of repeated low dosages (20-200mg per hour) throughout the morning and early afternoon for optimal performance. However, many of these studies have very small sample sizes and should be read cautiously.
I feel best with no caffeine at all.