Hey there! There's a lot of folks who structure generator hours in different ways. I would recommend taking a look at the listings in your area and see what strategy or charges your fellow owners have posted. Here's an example of an owner who gives a certain amount of free hours, and then has a sliding scale charge if there are overages-
I'm similar to Dave and charge $59 for unlimited on a WEN generator I offer. I used my local Home Depot to compare what they charge and then backed it down a hair. I also purchased the Wen because it's early easy to service and at just $800 investment won't be too hard to get a return on the investment while also offering a cheaper option to my renters than the closest alternative they are likely to comparison shop me against.