5 seasons seems to be the number
Falling Skies may have been a bit better at 4 but overall it worked very well. Flawed show but enjoyable and I was not wanting more but I was satisfied. Ill remember this show fondly. Fringe was another one that ended at 5. Bates Motel has already declared it will be 5. Lost was 6 and we all know it was one season too much.
So for those successful sci fi, suspense, horror, mystery shows, you know besides the 80% which get cancelled right away. Maybe 5 is the magic number