Any way, there's a guy in the area that needs 100 acres of weeds mowed down. He initially asked to rent my tractor and mower, but I don't do that.
Yup. Agree 100%. Also, an inexperienced operator might just roll the whole **** thing down a hill.
Any way, I told him I could mow it for him, but I'm not sure how long to estimate for it, and how much to charge. I have a 27 HP Kubota and a Deere 5' brush mower. I was thinking I could get it done over a weekend, but I'm not sure since I've never mowed that much land.
I have a 25 hp Bobcat and a 5' brush mower, so I'm pretty much in the same boat as you. I do about an acre an hour. I can go a little faster if the property is reasonably flat and has been hogged somewhat recently, but that is hardly ever the case for jobs like this. Usually, it's very overgrown. Because of that, you have to go much slower than a "mowing calculator" would suggest, because you're looking for debris in the weeds, finding boulders and rocks that you have to mow around, and so forth. Also, if the property is not dead flat, you are going to find spots that are steep enough that you may not be able to make the ideal approach--e.g. you have to drive up and down instead of across, or something--and that is going to further slow you down.
At 8 hours a day, that is about a 12-13 day job. I estimate that I burn about a gallon of diesel an hour (a little less, but it keeps the math simple). I also estimate about a dollar an hour towards my next 400-hour service. So that's maybe $4.50 an hour in cost just to keep the tractor running--bare minimum. Add in the potential for you to damage your tractor or mower, and I just round up to $10 an hour in equipment costs. Add on top of that what you think your time is worth, multiply by 100 hours, and away you go.
There are some on here who would say that the 1 acre per hour estimate is too conservative. IMO, it very much depends on the nature of the job. If it is head-high weeds, there is no way you can go at full speed, even if your tractor and your mower are big enough to handle the volume of material, because you are not going to want to charge blindly ahead. You may run into a ditch or a big chunk of cement or who knows what? If you had a very WIDE mower, then things might go faster, because you could still go at a safe speed, and cover more ground. I have only been bush-hogging for a little while, but one thing I have learned is that you seldom get asked to mow somebody's field that's been regularly maintained. Usually, it's some field that nobody has touched in six months, a year, or maybe more. The brush is 3'+ and you can barely see where you're going, and I just look at the mowing calculator that says I can do 3 acres an hour and laugh, and laugh, and laugh.