I figured once it says "get on the boat" you have safely 5 minutes of farm after that point. Anything after 6-7 minutes is unreliable and causes crashes regularly. Typically you'll start to see zombies sliding across the ground and most will just straight up vanish and waves will disappear entirely. 5 minutes is the limit.
It seems that if you stay in the final level for too long farming zombies the game will crash. I'm not sure if the game is designed to do this but it happens constantly. Basically in the end before you're supposed to board the boat if you keep letting zombie waves spawn the game will inevitably crash. Seems to happen more regularly once my kill count goes above 1000?