Health regained seems to be about one health per second.
It is usually best to accept this perk relatively early on in your run if given the opportunity as the loss of maximum health is less risky for a new character, but can be very dangerous for a stronger character. Also note that your health increase when leveling will be reduced by 75% as well, rounded down. If your character does not get at least 4 health per level, this perk will prevent any further natural health increases from leveling.
This is the only effect in the game that grants constant and unconditional health recovery.
Synergizes well with anything that increases maximum health or either prevents or mitigates damage received.