Healing Factor (3.5e Variant Rule)
From D&D Wiki
This variant rule includes more extensive information regarding one's healing factor, as well as rates of healing slower than fast healing but quicker than regular healing, as well as the ability to restore points of ability damage and even drained ability at accelerated rate.
Healing Factor (Ex): Humanoid ozhalig restore 1 HP per hour of activity or 1 HP per 20 minutes of rest, 1 point of ability damage per hour and 1 point of drained ability per day, and have a +12 bonus on Fortitude saves.
This description includes the rate the creature restore hit points and points of ability damage/drained ability and the bonus they gain against toxins and diseases (Fortitude bonus).
Of course, the description assumes the creature has 1 HD. If they have more, then the number of HPs they restore per the noted time is multiplied in the number of HDs they have, just like with ordinary healing.
Additionally, a creature may have both regeneration and fast healing, though mostly the fast healing's rate will be slower. This represents creatures that heal quickly even if wounded by weapons/effects that inflict them lethal damage, however still slower than they heal otherwise. Such creatures are usually very powerful, sometimes even epic-level.
Advanced Healing Factor: Overpowered vampires heal extremely fast; they have regeneration 20/magic, epic (+6), anti-vampiric, and fast healing 5, and restore 5 point of ability damage and 1 point of drained ability per round.