It depends on your membership.
If you have an active Month Membership, Boost will renew automatically each month unless cancelled. You can cancel at any time by heading over to membership & vouchers.
If you have a Sports Day Membership on your account, Boost will only run for one month, and won’t auto-renew.
If you cancel your last Month Membership (unless it’s one you pay for through iTunes), we’ll automatically cancel your Boost for you as well.
If you're billed by iTunes and you cancel your last Month Membership, you'll need to remember to cancel Boost with Apple as well.