This assumes there's no economy of scale to study time, and I'm just not sure that's true. It's not indefinite, because people tire out, but I'm pretty sure an hour long study session is more than 4x as effective as a 15 minute session.
It’s normally in the pop science articles that concentration starts bonking out around 40mins so I assume there are some studies backing that up. Personally I would take 7 x 15 minute sessions per week over a single 1hr 45 minute session or an hour session and a separate 45 minute session.
