Myself, i'm on the Insanity journey. At least 30 minutes (mostly 40 and a max of 60), 6 days in a week.
Week 3 since Monday, ps. 7 minutes is way to little in my opinion.
25 minutes is the minimum of cardio your body is going to need for "any change".
Now I'm doing 2 sets a day, so 14 minutes. I'll update you on the status soon.
But in my understanding, a minimum of 25 minutes is required for any (decent) change.
If you go from nothing to something, the first miles will always change you somehow... Although it might be only a little bit.
25 minutes is the minimum duration for your body to finish it's reserves :)