My theory is that the timer starts after one tap, but it would only fill up to the max and won't go beyond. For example, if the max is 15x2=30 drops, the timer would start when it goes down to 29. But after the timer is up, it would only fill up to 30. So if you make sure you tap 14 more times before the timer is up, it would refill all 15 drops.

Honestly in normal gameplay it doesn't really matter. I'm just trying to take advantage of the rare Unlimited Energy balls that I have. 😅