If all you have a day is 10-15 minutes, that's still enough time to make some headway. In fact, I've heard that working on one exercise for more than 15 minutes at a time can get counter-productive (this is, of course, subject to perspective).

My humble recommendation would be to focus on one thing, or one exercise during this time block, and really focus on it. For instance...10-15 minutes on just the C-scale (or some other scale); 10-15 minutes on a couple chord changes, the next day, etc.

I completely sympathize with your predicament, and lack of time/energy.