If you make it less than five seconds, the time frame between "Attempt Heal -> Player Blocks Heal" may be up to 2 or 3 seconds, depending on when they block the heal, which would allow a griefer to continually spam it. When you place a 5 second window, it at least gives the player an opportunity to get away. Anything less than 5 seconds really nullifies the usefulness.
Another thing you could do is simply allow players to use medpacks while moving. However, that really nullifies the negating value of using a medpack in the first place.