This site cites this specific source directly: PubMed, randomized trials of Bacopa monnieri and memory.
Worth noting this is a search-results link rather than a single study citation — it surfaces multiple peer-reviewed trials, worth browsing directly rather than assuming any one specific result applies to this exact formula.
This site's own Bacopa description cites trials showing modest improvements after about 12 weeks — worth checking this PubMed link directly to find the specific trials behind that stated figure.