My rule is to not have hard and fast rules. I have a variety of types of customers from weekly shop fronts, 6 weekly 'turn up and cleans', and others who ring me as and when they need me. I get a few 'not todays' from the regulars, and they tend to be the same ones who say it, but not all at the same time. I don't find that it restricts me as I have enough work to ride it. I could dump them, but I keep them on as long as they are making me money, and if they do it too often, I just put them into the 'ring me when needed' category.
In the winter I find it does me a favour for some to leave it for a couple of months. The days are shorter and I'm inundated with xmas cleans. In January the weather is poo and I'm wrecked after the xmas rush, so again, I am happy to take my foot off the gas and coast through, while still having enough to keep going through till March when things pick up in earnest again.
John