Not sure when the numbers kick in but the animation will end and start over if you just hold the button down. I do know that the refund of magicka and health come at the end of the animation. I use heavy attacks to fill my magicka faster if I'm in combat but don't feel I need a potion.
Edit: Left this out, If you don't let the animation end you won't get your extra boost of magicka and health returned to you. (Can't remember if getting these depends on morphs or champion points taken)
Charging it up all the way and letting it auto-release versus charging it for a split second and releasing just changes the damage, all your passives and magicka return and junk still happen, the damage is just a tad less.
LordSkyKnight wrote: »You need to complete the heavy attack to get a magicka return. That means a full charge before you release the attack. If you release the attack early you won't get any magicka returned.
Staves return magicka from a full heavy attack and melee/ bow weapons return stamina.