Andrew
I am sure, for many reasons, this varies, but the NCCN guidelines show in the first year that you should have an exam at least every three months.

My ENT said eventually I would just be seeing him. Our appointment is monthly for the first year, and he will order my PET/CT scan about every six months during the first two years, then at least yearly through the fifth year

In the second year, appointments will be every other month. then they become quarterly and at some point twice a year.

The NCCN guidelines do not seem to specify a PET/CT scan frequency after the post TX three month follow-up scan.

http://www.nccn.org/professionals/physician_gls/PDF/head-and-neck.pdf