I've always allowed instant or near-instant healing with three exceptions. 1, the wound cannot be cleaned via healing. That is to say, if the wound is dirty and you drink a healing potion, you now have a subdermal infection and a growing cyst due to it. 2, healing cannot revive the dead. The moment their brain shuts off, they're lost. You can certainly heal the wound, but you're just making a pretty corpse. 3, it drains the healer's ability (depending on the universe, their mana, tech power, etc. just need recharging) but it also drains the subject's life, since they only have [X] number of cell divisions, though this is explained differently depending on the universe. if you've got a large scrape on your thigh or a deep but narrow cut to the jugular, the scrape would be more taxing to heal as it requires more cell divisions than simply sealing a small hole.
Yeah... I go scientific with my magic. and healing magic is by far the hardest to reconcile scientifically.