It’s entirely up to you and what you think will work best with your fans, given what you plan to offer.
Monthly billing has the advantage of providing consistency to your income, and a lower price may be more effective for some fans (e.g., someone who might balk at $60 per year may gladly pay $5 a month).
On the other hand, monthly billing means paying credit card processing fees 12 times per year instead of once, and it may also lead to a higher cancellation rate as fans see the charge on their credit card statements each month.
If you’re in doubt, we recommend asking your fans.