I charge either full time which is 4/5 days a week, if they only need 4 they still pay for 5 because I can't fill a 1 day spot. After that it's considered part time and I charge a daily rate x's the amount of days per week needed x's 4 weeks =the total monthly cost.
Works great, that way they always have a set rate and if they are going to add to those days then I charge them the daily rate.