I have about 30 trees or more that I have cut down on my property. The biggest being about 18" diameter. How long does it take to grind a stump of this size with a typical rental grinder? Also after grinding ,how long does it take for me to have to come back with fill after the remaining stump/roots begin to decay? I may also have access to a John Deere 410 back hoe soon as I was thinking of pulling up the stumps complety. Using the back hoe I have been told this is a much bigger project than I could imagine....