Different brands have different lifespans. Mill running usage is possibly a lot of load on the tool . Depends on load, might not be very long. Or till the bushings or bearings wear out. Get brushless if you can , they are a wear item , get extras anyway.
I do have a battery grinder , it gobbles batteries . price in buying a bunch of them .
Some brands have chargers that will charge from a 12volt car battery/alternator .
I don't do commercial work anymore but DeWalt , Milwaukee ,Makita were the most visible on job sites no matter which tool.
I have a consumer Ryobi now , haven't used it that much but ,,,batteries is my hint.