I don't know the exact percentage of HP it eats at, but be assured you will never die. Once it eats the last piece of food in the inventory, it will log-out when your HP gets too low.
61-75 shouldn't take you too long. I didn't start Ape Atoll until 68, so I wouldn't know for sure.
I have 2 threads that may be able to help you as I reached 99 Agility not too long ago.