If the subscription billing cycle is specified when a subscription is created, then the subscription lasts for that period irrespective of the plan’s billing cycle. 

For example, if the subscription billing cycle is 5 and the plan billing cycle is 3, the subscription lasts for 5 billing cycles.

If the subscription billing cycle is not mentioned and is left blank, the plan level billing cycles will apply. For example, if the plan billing cycle is 5, then the subscription billing cycle is also 5.