Research suggests that a significant amount of hydrolyzed collagen resists digestion in the stomach and is absorbed directly in the intestinal tract. This is evidenced by high levels of gly-pro-hyp (a tripeptide) and/or other peptides in the bloodstream shortly after ingestion. What is not absorbed directly is further digested and absorbed in the form of shorter peptides or individual amino acids that are still highly beneficial (1)(2)(3).
If we refer to "absorption rate" as how much of the collagen peptides and amino acid constituents are absorbed overall, it's safe to say that it's likely close to 100 percent (humans absorb even non-hydrolyzed animal proteins very efficiently).
It is not possible to claim a specific absorption rate regarding "undigested" peptides; we can only say that it is a significant quantity and that marine collagen would have a higher undigested peptide absorption rate due to its smaller molecular weight compared to bovine, chicken or porcine collagen (4).
References:
Yazaki, et al., (2017) Oral Ingestion of Collagen Hydrolysate Leads to the Transportation of Highly Concentrated Gly-Pro-Hyp and Its Hydrolyzed Form of ProHyp into the Bloodstream and Skin, Journal of Agricultural and Food Chemistry.
Shigemura, Y, Suzuki, A, et al., (2018). Changes in composition and content of food-derived peptide in human blood after daily ingestion of collagen hydrolysate for 4 weeks.
Shigemura, Y, Iwasaki, Y, et al., (2018). A Pilot Study for the Detection of Cyclic Prolyl-Hydroxyproline (Pro-Hyp) in Human Blood after Ingestion of Collagen Hydrolysate.
Shigemura, Y, Kubomura, D., et al (2014). Dose-dependent changes in the levels of free and peptide forms of hydroxyproline in human plasma after collagen hydrolysate ingestion.
Sato, K (2017). The presence of food-derived collagen peptides in human body-structure and biological activity.
Sato, K, Asai, T, et al., (2020). Collagen-Derived Di-Peptide, Prolylhydroxyproline (Pro-Hyp): A New Low Molecular Weight Growth-Initiating Factor for Specific Fibroblasts Associated With Wound Healing.