How often do patients get scans. I think my dad gets them every six months. When is it once a year?

Thanks

Squamous cell / BOT with lymph node involvment, Stage 4, HIV-, cisplatin 3 weeks of 7, stopped due to kidney issues and neuropathy,35 radiation treatment started 5/6/09,started weekly erbitux on 6/2/09. Completed tx on 6/24/09, biopsy 8/11/09 clean, PET Scan 10/5/09 clear, PET Scan 6/11/10 clear