Remember folks who are currently subscribed: They do not give prorated refunds for full year subs. When you cancel the service ends immediately. Setup calendar reminders or eat the sunk cost now because Amazon Prime is just a typical subscription entrapment scheme that will only get harder and harder to cancel, like a gym membership.

Edit: Thanks /u/@firadin - Cancel now, you don't lose anything and the membership simply won't renew on your renewal date.