Automatically measure the total time the learner has spent on rehearsing each session, and in statistics show (among other things) the average rehearsing time per day. This would be very useful for time management. You can then commit for example to rehearsing at most 10 minutes a day.

Also, automatically make an estimation of the future rehearsal times based on past rehearsal-time data with the current set of active cards.

An additional feature would be that Mnemosyne would not accept to add new cards if the prognosis is that you will exceed a user-chosen average rehearsal time a day (for a reasonably chosen "moving average" window). Then you should deactivate some older cards, or wait until you have rehearsed the existing cards sufficiently. Of course, there may be some "variance" around the rehearsing time, the limit holds for the estimation of the average time. In addition, you could set some maximum "variance", so that, for example an actual rehearsing session is estimated never to exceeds n% of the average. This way you prevent big surprises in your schedule, for example that you suddenly have to rehearse for 40 minutes, even if the average is 10 minutes.

I notice that I tend to stop using mnemosyne, because at a certain moment it takes so much time to keep up with the schedule... The feature I mention could be a solution for this problem. It warns you in advance so that you do not end up with too many cards to fit your agenda.