there are many video clips now at 60fps especially in HD.
my experience is that I did not have any increase in sales from selling clips with the speed adjusted.
I do not have any additional sales from my 120fps clips.
in terms of length, I did not have any additional sales from longer clips. for example, a 10s clip of traffic vs a 30s clip of traffic. the only exception is any clip that might be suitable as a screensaver. for those, I have been able to sell 120s clips.