just found this after defeating monstro on cellar 2 fyi
" Isaac regenerates a half-heart every one minute of gameplay. " This cannot be. I am using the in game timer and it sometimes will regenerate a half a heart every minute. This must be a % chance every minute to regenerate half a heart. ^It appears that the longer you stay in a room, the % chance decreases. I found this out by attempting to get the Super Meat Boy and The Bandage achievement using the Placenta, the Habit, the Monster Manual, and the Epic Fetus items. It began taking 40 minutes to regenerate 6 health.
-Another User This may indeed be true, as I have seen the same results myself. As the half a heart a minute regen begins to shows signs of decreasing after a few minutes.