Forget the binary; the duration of a trip is not the only factor in enjoying the low season at lower cost, it’s a matter of flexibility and destination choice.
Neither two weeks nor three; in Japan, we don’t just think in terms of duration, but also public holidays and local festivals.
Traveling just before Golden Week, for example, allows advantageous rates and less crowded conditions, even for a longer stay.
It’s a matter of precise timing, not just number of days.