I generally did around the 15 minutes mark like you did and I noticed unsatisfactory results. Fatigue, feeling blah, etc. I moved back down to a minute and worked my way up from there to the 2 minute mark. These LEDs are super powerful on the Joovv, and I think that it's way easy to overdo it and end up with too much of a good thing.
- - - Updated - - -
I notice a lot of improvements from the vision side of the house, myself. I also like to do what you mentioned, and did on my last Ultra Hard summer cycle and I noticed no suppression symptoms whatsoever until the very end of the month.