I've taken Provera to induce a period once this summer, and my period started about 2 days after I took the last pill. If you're really worried about the withdrawal bleed showing up over the Christmas holiday (when labs won't be open), just delay it. There's no golden rule that says that the pills have to be taken starting on CD 60... in fact I took mine starting CD 104.
However, like PP's have said, some people react differently to the medication and their bleeding starts before they have finished the course, or up to two weeks after stopping the medication.
Hope this helps and good luck!