How long should you study spaced repetition every day?

For the last month, I have been Net matured spaced repetition cards cards at about 3.3 per day. and learning cards at 2.9 per day. I happened to average adding 1 card per day over the last month. While my review times were all over the place, the were trending slightly down below the 15-20 min max I want to have for a lifetime commitment (I think my ideal might be 10 minutes: How long should you study spaced repetition every day?).