Probably didn't take long to grind yours. My son has a stump grinder but real small the size of a lawn mower but still pretty aggressive. I would guess that big one would not even bog down.
He said nothing stops it. It probably took her 2-2.5 hrs to get it all done. Just guessing mind you.
Going to be sunny and 16 today and supposedly a lot of rain tonight into tomorrow morning. Just might go and buy that battery today to be ready. I took the cover off partially yesterday to show a guy the car and is nice and clean so looks like put the battery in check fluids and tire pressure turn the key and go.