Why there is no number
The honest answer to how many hours a course needs is that it depends on what you already know, how the material is assessed, and how you spend the time — and those vary enough between two students in the same class that an average would be misleading for both. The common rule of thumb about hours per credit is an institutional planning convention rather than a research finding, and treating it as one gives it a precision it does not have. What the research can tell you is comparative: for a fixed number of hours, some arrangements produce more retention than others, and some activities within those hours produce more than others.
Arrangement beats duration
Cepeda and colleagues found distributed practice beating massed practice at the same total study time, with the advantage widening as the delay before testing grew. That is the most useful sentence in this guide: the same hours, spread out, are worth more. Within a session, Roediger and Karpicke found that time spent retrieving beat time spent restudying at any delay beyond a few minutes. So two levers exist before you add a single hour — spread the sessions, and spend them testing yourself rather than rereading. Both are free, and most students are using neither.
How to tell whether it is working
The feeling of having studied is a poor measure, because fluency rises with exposure whether or not retention does — which is why rereading feels so productive. A better measure is whether you can produce the material without the source in front of you, several days later. That is what a practice test measures, and it is why the Practice-Test Ledger records what you predicted alongside what you scored. The gap between the two is the useful number: someone consistently predicting well above their score is misreading their own preparation, and that misreading is a bigger risk than the hours.