The longer he is around someone, the longer he can keep the power once they are out of his range. It is not stored more than a few moments in any case. There's not been a limit shown to how many powers/skills he can dupe at once, but I can count how many he had at once in his 1 PC appearance as opposed to his Post Crisis one. His power works the same in both, and the PC appearance is brought up and shown in a flashback-still during his Post Crisis showing.