How long ago was the surgery, how long was your adjuvant chemo, how often will you be getting scans and ctDNA tests, and what have your CA19-9 levels been since surgery and since adjuvant chemo ended?

Were you declared "NED" based on surgical results + imaging + CA19-9 + Natera?

I'M NOT A MEDICAL PROFESSIONAL, but I guess it depends on how well you know what your typical CA19-9 values are, and how stable the doctors consider you to be. However, being only 1.5 years out from the end of your chemo, I would consider 4 months a bit too long for comfort as well. A LOT can happen in 4 months, and the slowly rising CA19-9 actually signaled the beginning of my post-Whipple recurrence before imaging or Signatera ctDNA tests did.

(I went from NED for 3 months after a "perfect" Whipple to recurrence spotted on MRI at the 4.5 month post-op mark, with EUS/biopsy and 2 Signateras being negative, before another Signatera barely registered positive 7 weeks later and MRI confirmed mets along with the primary tumor growing. So, basically 6 months from Whipple to Stage-4. CA19-9 in that interval went from 12 to 33 to 77 to 277, and then 678 in 6 more weeks before I resumed chemo.)

Now on biweekly chemo for the recurrence, I get CA19-9 tested every two weeks along with CBC/CMP and the others necessary for another round. My last (local) oncologist would only test every 4 weeks while I was on my bi-weekly pre-op chemo. I was a bit frustrated by that, and got my primary care doc to write me an order for repeat/serial testing that I could take to a walk-in lab myself and self-pay ($25). Cheap enough for the peace of mind... So I was getting tested every two weeks, but at different labs. Their scales were different so the results were not directly comparable, but by keeping them separate for analysis, I had a better picture of the upward trend from both labs telling me the pre-op Folfirinox wasn't really doing much.

If you're starting with a new oncologist and a new lab, I (personally) would try to get tested every month to understand your normal values and trend, and then see how your new doctor's results match up.

But to be honest, if you've already had PDAC, I would be looking for another specialist closer to home. This one doesn't sound very aggressive, and I would worry as much about his/her response time as much as detection time.