I read about this a little when they found my lung met. Lung met doubling time for CRC can be as little as about 30 days, which it was in my case. Here's a random paper where they find a median doubling time of about 66 days, but a pretty wide range: "Impact of Surgical Resection on Metachronous Metastases of Colorectal Cancer According to Tumor Doubling Time" Miyake et al.
What doubling time you may be seeing depends a lot on what the starting point is. What was the sensitivity of your old scan? If some nodules were 2mm then (easily overlooked), and 6mm now, that is an increase in volume of 27x, or about 4.7 doublings, which would be a DT of 38 days. If your old scan was completely clear, not even a 0.5mm hint of a nodule, then that was over 1728x increase in volume, or over 10.7 doublings - pretty extreme for 6 months.
If you want, you can look at your own scans. A 6mm nodule on your new scan should be pretty easy to see on a CT. Then go look at the same area on your old scan and see if there is /anything/. In my case the lung met was actually visible on previous scans, but the radiologists had reported it "clean" because the nodule was so small.
I hope yours turn out to be nothing!
Male 42 — stage IV RC
NRAS mutant - KRAS, BRAF wt
08/2019 DX 6 cm long tumor
12/2019 TME Surgery, clear margins, 7/16 nodes positive
Pathology: ypT3 ypN2b M0
01-06/2020 - FOLFOX
CEA only goes up during chemo: 2.4 --> 6.2
07/2020 6 mm tumor in lung, was growing fast during chemo
01/2021 new 5mm cyst in liver, CEA continues to increase --> 8
06/2021 CEA takes a break - down to 6