What is the length of a frame in Final Cut Pro in seconds? Possible to specify it? It took 23 frames and 0.92 seconds for the ball to travel from the first picture to the second so one frame is 0.04 seconds long, with 0.1 slowness (so the speed of the ball is roughly 150 km/h). How can I specify the length of the frame? Is it possible to specify arbitrary precise frame length in time?
The length of a frame is proportional to the frames per second of your clip.  

Professional films, and HD television are typically set with a base frame rate of 23.98fps.  So 1 frame will be 1/23.98th of a second.

Further details on specifying and adjusting the base frame rate in Final Cut Pro can be read indepth here:  https://documentation.apple.com/en/finalcutpro/usermanual/index.html#chapter=D%26section=4%26tasks=true