I like to walk a property before quoting. Up here in NH, there are no big open fields, just small, rocky, wooded, steep patches. I average about 1/2 - 1 acre per hour. My first cut is almost always 1/2 and within a few cuttings up to 1 acre per hour. Cutting varys based on customer. anywhere from once per year to 3 times per year.
I have a customer with a pond that I cut once per year. The property is 4 acres divided into 3 fields and is loaded with obstructions. Takes me about 6 hours now. But I back up to the water's edge with the mower and cut as far as I can into the water. Lots of back and forth and maneuvering.
A Ventrac or Steiner is going to cut about the same rate as my 5ft brush cutter and 25HP tractor, on the flats. If it were me, I'd use a cut rate of 3/4 acre per hour. So 15 acres / 0.75acres per hour = 20 hours.
Can you limit the number of cuttings in the contract? Add an assumption with a per visit rate?