From Diablo Wiki
Replenish life allows you to have a small regeneration of your life when you are hurt, but it is small indeed so keep in mind that you will need a heavy dose of replenish life to be actually noticeable in battle. The amount hasn't quite been registered to quite understand how much health is regenerated per second, or minute, but it regenerates health nonetheless.
Replenish life is good in having a small but decent life replenish when you don't want to waste so many healing potions to heal, but like described above, you need a heavy dose of it to actually replenish an abundant amount. Replenish life works much like Replenish mana but it activates that replenishment other than stacking upon the mana that you can already replenish. For people who are poor and in need of some small backup, replenish life will be your best friend.
The following formula may be used to determine the amount of life gained per second. However, this formula has not been confirmed, and may also be out-of-date.
( ReplenishLifeAmount / 256 ) * 25 = LifeGainedPerSecond