Well if you really want a by-the-book answer, then you should be playing around 1000+ STTs for every level to get enough of a sample size to really determine if you can beat them.
I think if you're starting out with the <$10s, then playing in the 1000s isn't needed. They're pretty easily beatable, and after a hundred or more you'll know if you can beat them.
As you move up in levels you should probably take more time at each stake since they'll get harder and harder, and you might not know if you're truly beating them or if it's just variance.
I was running at %60 ROI when I moved to the $16 turbos at Stars after a few hundred games, but that soon came crashing down before I could move up. I'm now at a healthy %15