12 months or 24 months
You pay the entire amount when you sign up for a multi-month plan (it’s a one-time payment). They’re giving you a discount for signing up for a longer term plan (that’s what the per month amount is, but you pay it all up front).
If you’re not sure you’ll stick with it, you’re smarter to stick to the monthly plan.
Yes, your account will remain Premium until your paid period expires. It won’t drop to Free level in the moment you cancel.