You should decide the length of a keyword but try not to define a keyword as just one word. It won’t be helping you to be resulted as first on the search engine. For instance, if you choose “television” as a keyword, that would be affectless. It would be very general and there would be millions of results for that. The interactions would be gained by the widest firms and websites in this way. So, it is almost impossible to reach success by the word “television”.  Try to use keywords such example “the best TV series on television” which will decrease the competition rate and increase the interaction since it is very specific.