It's by hit counts. Most EX moves that provide armor provide 1 hit. You are afforded 1 hit before you get knocked out of your move. For "time active," you're probably thinking about invincibility frames, which is a predetermined number of frames (say 10-13) that you can't receive any damage whatsoever.
1. Armor= a predetermined number of hits you can take during a move.
2. Super Armor= so long as it's still within "super armor frames", you cannot be knocked out of the move.
I'm pretty sure I have the right idea haha Might be dead wrong though