The American standard for decaf coffee is to remove 97% of caffeine content[0]. (Note - I couldn't find specific CFRs on an official government site, so it's possible this is just a recommendation and not actually enforced. However, a quick google will confirm that this is the consensus on how things are supposed to operate - I make no comment on how they actually operate.)

[0] https://en.wikipedia.org/wiki/Decaffeination#Caffeine_conten...