Exept for the fact that in order to turn you into candy he allready has to kill you.
You only turn onto candy when you run out of HP.
It would seem that when Buu uses Candy Attack, the needed distance for the attack to succeed depends on how strong the oponent is.
Candy Attack could have some charge time and that “thing” on Buu’s head could go to front so the opponent could see that Buu is going to use Candy Attack.