We do not require you to sign up for a set number of months, however, we do offer discounts for paying yearly (12 months). You are free to cancel your recurring payments at any time on any of the plans.
When you cancel your subscription, your account is not immediately deactivated. Instead, your account is active until the end of your current billing period. So if you signed up for the quarterly billing and cancel after 6 weeks, you still have access to Linux Academy until the end of the three months.
See here for instructions on cancelling your account.