I just stumbled across a good paper with some good references on growth rates. I can't write a proper summary at the moment, but include some quotes and links below:

The paper: https://pmc.ncbi.nlm.nih.gov/articles/PMC4520782/

Quotes:

"Evidence based on mutational analysis predicts that pancreatic cancers are confined to the pancreas for many years, perhaps a decade or more before spreading beyond the pancreas."

"Clinical experience indicates that most patients with advanced stage pancreatic cancer progress rapidly."

"We find, after adjusting for factors associated with age at diagnosis, the average age differences of patients with larger versus smaller T stage tumours is small and often not statistically significant, suggesting that most pancreatic cancers rapidly progress from lower to higher T stages."

"We estimate that the average T1-stage pancreatic cancer progresses to T4 stage in just over 1 year."
[ Good reference on tumor sizes (T1-T4) using the TNM scale at: https://www.cancer.org/cancer/types/pancreatic-cancer/detection-diagnosis-staging/staging.html ]

"This undetectable period of growth may represent the majority of the overall time to progression, since a founding cancer cell has to undergo many tumour doublings to become a detectable tumour (a 1 cm diameter tumour contains ∼1 billion cells), but only a few more doublings to grow into an advanced-stage tumour. The most relevant timeframe for early detection purposes is the time from when a cancer first becomes detectable by diagnostic tests up until it begins to progress beyond stage I disease. Although pancreatic cancer has a poor prognosis overall, patients with small stage I cancers have a better outcome than most patients with resectable cancers (mostly stage IIB)."

"Several studies have estimated the growth rate of various cancers using serial tumour marker measurements or serial CT scanning,21 but only limited data of this kind is available for pancreatic cancer. One study that measured serial serum CA19-9 and carcinoma embryonic antigen (CEA) measurements in patients with advanced stage disease estimated the average tumour doubling time of pancreatic cancers to be ∼40–60 days."
[Reference below]

"consistent with the hypothesis that once a pancreatic cancer is detectable by diagnostic tests, its growth and progression to more advanced stage disease is rapid. "

"We also found that larger primary pancreatic ductal adenocarcinomas were more likely to be poorly differentiated. There is evidence that tumour hypoxia which is likely to be greater in larger tumours contributes to the differentiation state of the tumour."

"In conclusion, we find using the adjusted average ages of patients with localised or locally advanced pancreatic cancer at diagnosis that disease progression is rapid, with an average estimated time of 14 months for a T1 pancreatic cancer to progress to the T4 stage."

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Reference for the "~40-60 days" tumour doubling time:
https://pubmed.ncbi.nlm.nih.gov/10404129/
Results: The doubling time of CA 19-9 and CEA could be calculated in 90.2% and 58.5% of patients with inoperable pancreatic cancer. CA 19-9 doubling time was clearly associated with survival time in inoperable and palliatively operated cases, but not with sex, age, site of the lesion, or liver metastasis, and was significantly correlated with the tumor volume doubling time.

Conclusions: Examination of CA 19-9 doubling time may be useful in clinical evaluation of the prognosis for patients with pancreatic cancer and could possibly prove valuable in terms of the analysis of the growth process in this disease.