Sustained focus over long study sessions is a specific need worth reading against this site's own focus-and-concentration mechanism front.
This site describes L-Theanine's role directly as promoting calm, focused attention — worth reading that mechanism specifically against a study-session use case, distinct from the memory-formation mechanism relevant to exam recall.
This site is direct that focus-related benefits are expected within the first few weeks — worth planning around that window well ahead of an exam period, rather than starting the routine the week before.