Khan Academy. Finish a course, retake the course exam once a year afterwards, fill in knowledge gaps as needed. Doing this should keep all undergraduate math fresh in your mind.

I wanted to like Khan Academy, but the pacing of the exercises is so frustrating. They become too easy too fast and then it's an extremely repetitive slog of useless computations.

I wish for a service just like them but with many more difficulty levels and less (non-spaced) repetition.

I recently came across brilliant.org, which I think aspires to be this. I didn't stick with it but it might be what you're looking for.

How about a textbook? At 15 mins a week, a couple of books would last a year.