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