Yeah, it's the same for all characters, it doesn't have anything to do with how many times you go into power-up mode. The counter goes down faster or slower depending on if you're using shot, laser, or lock on. Tap shot (or press nothing at all) for the counter to go down slowly.
According to the DS booklet (page 12), if you press no button or tap shot, the counter reduces by 1 every 4 frames. If you hold laser or lock-on, it reduces 1 by every 2 frames, so twice as fast.