Bad question, because duration itself does not define the seasonality of a stay.
Instead of focusing on length, you should look at date flexibility and the ability to anticipate specific "quiet days," which is a gymnastics many cannot afford.
Here in Japan, a well-placed long weekend after a national holiday can be more advantageous than two weeks in "low season" when everyone rushes there.
It's the finesse of the schedule, not the length of the trip, that really determines advantageous rates.