Spectra S1

I just got the spectra S1 & I had a question about the rechargeable battery. If it’s 3 hours does that mean if I go out for 3 hours it’ll be dead by then or do those 3 hours only go for when the pump is actually on & in use? Sorry if you that doesn’t really make sense. I go to school from 9-1:45 so I’m wondering if I’ll have to charge it between classes or will it last me the whole school day? I pump once around 11.