Normally it is a combination of load, weight and time, they (Waste Management) give you a certain amount of time on the rental for the load, you just have to call them.
Depends on what your putting in it. The norm is a week for x amount. Your charged the same weather you fill it or not so dont order bigger than you need. Than again some will charge for overfilled i.e. 25 yards in a 20, you should have ordered a 30. Than theres what you put in it. General trash or M.S.W. "mixed solid waste" is garbage. Construction waste or "demo" is cheaper as they recycle alot of it. Yard waste, dirt and concrete are all differant than either of those and should not be mixed in if you have large quantitys. Also things like tires appliances tv exc... tend to have a extra charge. Be specific when you get a qoute to avoid over charge
Edit: this may vary based on who you order from, get WM's price then get a smaller company to beat it.