Since they area often less than 5 days and not going to be there at all I would probably charge them for 3 days.

If there is a set schedule you could add up all the days in the 6 week rotation and divide by 6 to get an average or use the previous month to get an average. But I think 3 days is fair since it is free money in the sense of no expenses on your part and they are often a 3 day a week family. It sounds like 3 days is their norm but some weeks they have an extra day.

This is often one of the reasons we charge parents a set fee for part time equal to 3 days plus a bit and then also have an extra fee they pay for any day over that. Then on a vacation week it would be their normal 3day plus a bit rate.