Cancelling a subscription only means that it will not renew when it reaches the end of its duration. Regardless of whether you have paid for a week, month or a year, you'll always keep your perks up until the subscription ends.

In practice, this means that if you buy a 1-year subscription on the 1st of January and decide to cancel your subscription the next week:

1) You’ll still have access to the subscription perks until the 31st of December
2) You won’t be charged another yearly subscription at the start of the next year

If you have any further questions about subscriptions and charges, you can contact your current device's app store directly (Google / Apple).