It will adjust the charge current depending on what state of discharge the battery is in. The current drawn is dependent on the battery. It’s an up to that max amount of 2.5 A. The charge current can be a lot lower than that depending on the battery connected.
A smaller MAH rated battery should reach full charge faster. It will also discharge to a low battery state faster than a bigger battery. The bigger the MAH rating the more capacity the battery has, it will run longer at that same current draw.
A Pi Zero isn’t going to draw a lot of current so you should get decent run time.
It’s about 100ma at idle.
A 500 MAH battery should last 4 hours + at 100ma draw.
It’s going to be more than 100ma with the Pi doing tasks and that drawn by whatever else you have connected to it. The more current you draw from the battery the quicker it reaches a discharged state.