I do four weeks every year.....

That said I give parents 10 weekdays per year to use at thier discretion, they may use it for thier vacation or mine, but they must pay for the remainder of my vacation when the ten days are used up (for full time attendees) and I must have one months notice for any days that they wish to use in that ten day allotment. For pt attendees I give 5 days for thier use and the same polict applies.

Haven't had a problem yet.