My 91 year old father had MOHS surgery to remove squamous cancer on his face, his cheek. He first went to the dermatologist for it in February of 2012, last year. The lab did not detect cancer at that time. But it had an infection as well and we kept coming back and eventually they took another bioposy which detected it. It was operated on July 8, 2013 and we were told it had grown quite a bit but that it was completely removed. But she was concerned enough to suggest a CAT scan of the head and neck to see if the cancer might have spread to the lymph nodes.

I'm not very confident in the judgement of the two doctors at this practice who have let the cancer go so long without a second test. [I'm also a bit dubious of their judgement that the cancer is in fact fully removed, despite the MOHS good reputation.]

My question is: when do think would be the optimal time to take the CAT scan and if you think that is the correct way to proceed? They seem to want to wait another 3 or 4 weeks to take the cat scan.

I won't comment on the lapse of 17 months between his first visit and the surgery.
Squamous cell does have a very low mortality rate, but it does recur, especially in areas of the body exposed to the sun such as the face, and it often recurs where it first occurred.
If it's going to spread to lymph nodes, it does make sense to wait before doing a scan. It doesn't make sense to do it sooner rather than later, and in fact 3 - 4 weeks is pretty soon, and he may need future scans, even if clear.

(My father had melanomas and non-melanomas all during his 80s, and lived into his 90s and died of other totally unrelated problems, if that's any consolation.)