I assume you want to allow users to choose how often they pay for a subscription?
Currently the only way to do this is to set up multiple subscriptions with varying durations and relevant price adjustments ($50 for a one-year subscription, $25 for a 6-month subscription, etc.). However, because SMF sees each one as a separate subscription, users could choose multiple subscriptions and throw things off (if they chose both a 6-month and one-year subscription, they'd pay $75 initially, $25 in 6 months and another $75 in a year...).