Yesterday I used it from when my device was at 97% until it was around 40% or less. With over 4 hours screen time.
I stopped working because I was exhausted. But I assumed it could go at least 1 more hour if I kept using it until 25% or so.
For me this is a lot because it takes nearly 4 hours for me to charge my tablet… 
How about yours?
Not very long. It is very battery hungry. I have a 2021 M1 ipad pro and have to have it plugged in.
As I’ve documented on a flight with my 2020 iPad Pro (A12x = M1 predecessor), my accumulated sessions might’ve totaled at least 5 hours (accounting for additional usage time at the gate). As the cabin was overall dim, I didn’t need to crank my brightness at all. That likely helped the tablet conserve battery power.
With the upcoming M3 iPad Pro, it’ll be entertaining to see whether the chip’s 3nanometer manufacturing process will help its efficiency… whatever the case, it’ll be offset against the thinner 5.6mm to 5.0mm redesign (likely less battery capacity) and might be affected by the change-over to a double-stacked OLED setup. In a general OLED vs LCD query, some are suggesting OLED tends to be more power-efficient, but in the new iPad’s situation, there’s TWO stacked OLED screens for bringing brightness closer to what LCD/microLED can achieve.