Can I buy a 30, 60 or 90 day subscription?

Would that entail singing up for the monthly service then shutting down at any of these intervals? We only take registrations 2 per year. Tx.

Yes, you can sign up for a monthly subscription and just downgrade back to "Free" plan if the event or registration is over.

In your case, if you just want it for 1 month, then just downgrade before the next billing date will happen. If you want it for 2 months, then just downgrade before the 3rd monthly charge, and so on...

Here's a guide on how to upgrade to a paid plan: https://www.jotform.com/help/331-How-to-Upgrade-My-Account 

Let us know if you need more information on this.