Subscription duration is determined when setting up the pricing option for a membership pack.
Three settings control subscriptions:
The pricing option needs to be set to Recurring
Membership expiration is absolute and can be any number of days, months, weeks or years
Billing interval, similar to expiration can be set to any number of times in days, months, weeks or years
A common monthly recurring subscription would be as per the below example, expiring 1 month and renewing in 1 month - the billing interval is essentially when the subscription will auto-renew.
Subscriptions are active and available to the member from the date of purchase.