I finished up a similar project about three weeks ago. AA Rentals here gives you the Vermeer Hyrdasweep for one day, 8 hours on the meter. Makes for a long day in the hot sun.
Anyway, I had about 32 stumps, and three of them were from enormous, multi-trunk big leaf maples. The first stump took 7 hours and produced about four yards of chips! It turns out that the stumps take exponentially longer to grind as they get a little larger, because I finished all 32 stumps in 16 hours. When the arborist quoted me $1500 to grind these stumps my eyes bugged out, so I guess I'm a little wiser and humbler now.