Good choice in hiring it out. I just cut 3 smaller (12-18") cedar & spruce down the other day and the individual ground them about 12" below in less than 30 mins. Although your's is substantially larger, they won't have to re-position much, so it shouldn't take long at all with the right equipment.
HUGE stump grinder FTW! I dont' want to guess on what this thing weighed, but it had one tire on top of a 1" thick piece of slate for a few minutes and it snapped the slate into 4 pieces! Serious piece of machinery. I'm very glad I got to watch it, but also very glad I didn't operate it. Took him around an hour and all I had was a giant hole filled w/ mulch. Spent most of the day yesterday tamping, setting the form and pouring concrete.