I also charge monthly....throughout the year. Flat rate for mow, edge, trim, and blow. I mow EVERY week in the growing season, even on 5 week months for the same rate (this leaves room to play with when the grass doesn't grow). Soon I will be cutting every other week. In some cases, it will be every 3 weeks. But my rule of thumb is that I will edge and blow at least once a week until the edging shows no growth at all. This way I am still going by the property weekly until the growth truely stops. I will have a few customers come up to me during slow growth and ask whats up with no decrease in their bill. I simply state that I give them 5 cuts for the months that contain 5 weeks without charging extra, then I come by every week to see if it needs to be cut. If you over-cut, it will compromise the health of the lawn. After all of that, they usually have no doubt that I am fair.