As for skin, 12 weeks (3 months) is a common duration according to robustly designed research, assuming you're taking collagen daily. This 2019 study, for example, found that participants taking a collagen supplement (which included collagen peptides, acerola fruit extract, vitamin C, zinc, biotin, and a vitamin E complex) experienced enhanced skin elasticity, hydration, and density after that time period, and another with a blend of collagen and antioxidants found that respondents experienced smoother skin texture.* Additionally, this 2021 systematic review and meta-analysis of 19 clinical trials from the International Journal of Dermatology found that hydrolyzed collagen supplementation is effective for improving multiple dimensions (hydration, elasticity, and wrinkles) of skin aging after 90 days.*
That’s not to say you won’t be able to see results sooner. One 2018 randomized controlled trial measured skin-related outcomes (such as hydration, elasticity, and wrinkles) from low-molecular-weight collagen peptides at both six and 12 weeks—skin hydration was apparent after those six weeks, but participants had more significant results after the latter.*
Another study even said they found statistically significant skin elasticity enhancement from hydrolyzed collagen peptides after just four weeks, but the results were even greater at eight weeks.* Finally, this 2020 study measured the skin markers after taking a nutrition supplement containing 10 grams of collagen peptides at two, four, six, and eight weeks—with noticeable results at six and eight weeks but not before.*
That was a lot of science speak (told you collagen had a noteworthy breadth of clinical evidence!), so what does this mean for your supplement routine? Well, you can start to see results after the second month of use (8 weeks), but if you want full results, you should stick with collagen until at least three months (12 weeks). Again, that's assuming you're taking the collagen daily, but then you can assess whether or not it’s actually working—if it’s not, you may want to double check you have the proper dosage (research points to 10 to 20 grams per serving), a clean, high-quality formula with clinically backed ingredients, and the best types of collagen based on your needs.