I did a quick search and it appears the law sets a minimum of about $20 per hour. So I guess DoorDash considers there to be about 40 minutes of "active time" per hour?
There were exceptions I also saw, someone at 50%, some other people said they were at ~90% although I didn't see screenshots.
I assume this varies heavily by if your in a city or not or how close to meal time it is which I've got no insight into.
And note this could be all entirely wrong - it's people who chose to post their screenshots online so huge risk for selection bias to it being higher paid workers.
So if you schedule two hours and the app give you no hits, or you decline hits that time wouldn't count.
Now that said, I know nothing about the $20 min wage and can't say if that's accurate or not.