What is the length of a frame in Final Cut Pro in seconds? Possible to specify it?
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