There is no magic number — A short study is not automatically weak and a long study is not automatically strong. Duration has to be judged alongside the intervention, comparator, endpoint, sample size and the claim the study is being used to support.
Match the clock to the outcome — If an advertisement makes a long-term growth or density claim, a study should run long enough to measure that outcome meaningfully. FTC health-products guidance treats sufficient duration and appropriate endpoints as part of adequate substantiation.
Hair outcomes can take months — As one treatment-specific example, the American Academy of Dermatology notes that minoxidil can take six to twelve months to show results for some people. That does not set a universal rule and does not establish a Folivis timeline; it illustrates why study duration must be matched to the outcome and intervention.
What to check — Look for a prespecified endpoint, the timing of each measurement, the number of participants who completed follow-up, whether the comparator was appropriate, and whether the published conclusion stays within the study period actually observed.