Mrdi,

Not trying to be sarcastic or funny ... According to who and at what time?
Early 1900's - Theodore Gordon's personal favorite rod was a 9 foot Leonard (weights were different and differently designated then).
Latter 1900's - Lee Wulff prefered a 6 or 6-1/2 foot 5 weight.
If there were a 'standard' today, I'm guessing it would be somewhere in the 7 to 8 foot range.

Allan