A month in Japan - 21 day rail pass? Or less?

Hi,

I'm still figuring out the length of time I will stay in Japan, although I'm arriving in a week's time. My tentative route is Tokyo - Fuji 5 lakes area (to see Cherry Blossoms)
Nara
Kyoto
Beppu
Beppu - Tokyo

I'm thinking I could stay a month in total, and I want to minimize my travel every day. Would it make sense to get a 21 day rail pass if I stay in the country a full month? Or if I stay three weeks with minimal day trips?

Thank you,
Tanya