When I did my SS progression, the minimum duration interval I did was 20 mins (I started with 2x20). My logic was that I wanted to build endurance and TTE, so longer intervals better than shorter.
My goal for the intervals was also to keep my average HR no higher than 83% of max (based on guidance from the coach Steve Neal).
I started with 20 mins at 85%, and pretty soon was doing 20 mins at 90%.