Right now, with the grass growing fast, 8 hour days. As said above though, that's roughly one hour cutting, one hour charging, resulting in 4 hours cutting per day. Bear in mind though, once the lawn is in shape, the mower has a VERY easy time of it. It simply shaves off a tiny amount daily.
I also fitted a TP Link smart plug in the garage to the power supply. It means that if I know the weather is going to be REALLY bad, I can use my phone to kill the power to the dock before the working day. The mower will then stay put until the mains power comes back on. It will cut ok in the rain, but if its a really bad day, I'll keep it docked.