What is the Average Time Between Purchases for a given Subscription Cohort?

To get a quick look of the average time between purchases you should start by selecting "Days Since First Order" from the menu.

Menu > Cohorts Retention > Days since First Order > Segment by Subscriptions > Choose: Subscribers or Non-Subscribers

From there, if you segment by Subscribers (or Non-Subscribers) you will see the averages per cohort.