You asked a question that most everyone would ask, Is there an optimum Length for the air horn. As you may have guessed by now, there is NO simple answer to your question. The Optimum Length of the air horns, as stated earlier, is not only RPM related but dependant upon any modification you may have already made or would like to make.
Some of the answers you desire will be found by reading "between the lines" of other posts.
There are equations to calculate intake track length, sometimes trial and error is easier. But as Jim said, longer gives better low end and shorter gives more RPM. In some of the threads, exact length may have been listed. But that info will only be usefull if you know their engine modifications.
However, someone one maybe able to give a good recommendation based upon their personal experience.
Good luck in your research.