Short version: If you offer users a discount for paying yearly instead of monthly, why not generalize that and give them a slider to choose how often they'll pay. Every 2 months, every 6 months, every year, every 2 years, every 5 years, ... You can even let them pay once for a lifetime subscription. We made a little demo of it and give the equations for computing the discounts perfectly fairly for any payment frequency.

It's pretty great (if your users happen to be huge nerds)!