Hey there! Sorry for the late reply. You are correct, the game keeps track of time even when the game is closed.
I am actually looking to add that exact feature in my 1.1 update, for that exact reason :D
So in my testing it was anywhere from 14-20 hours. It'll never give you the battery life of just having the playdate suspended, but I run at a very low limited frame rate, and do updates very infrequently. I tried a good amount of other methods but they ended up adding little to no extra battery life for what was being given up, and in some cases used more battery.