Only if you let them, and it is off by default. Turn on Edit delivery frequency under Settings → Customer Portal → Subscription actions and a pencil icon appears next to Delivery frequency on the subscription in their account. They pick an interval, days, weeks, months or years, and a number. The number is capped at 12 for days, weeks and months, and at 1 for years, so there is no "every 18 months" and no "every 2 years". The change applies to the whole subscription from the next order, and the next order date is recalculated to match.
Two exceptions. A Club subscription refuses the change, because a club runs on a fixed billing schedule everyone shares. A prepaid subscription should be left alone: changing the frequency rewrites the billing schedule and breaks the prepaid arrangement the customer already paid for. Subscriptions billed through Mollie are not an exception any more: the same switch governs them and their customers get the same control. "Choose what customers can do in their account" lists every switch and its default.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.
