Every feature has a set duration, shown clearly at the point of purchase, before any payment. The price and duration are displayed together so you know exactly what you're getting.
Throughout the period, your ad stays boosted automatically, with nothing to do on your side.
When the duration ends, your ad simply becomes a regular free ad again: it is not deleted and stays online as normal. At that point you can choose to feature it again if you wish.
Featuring does not renew on its own: there is no hidden subscription. Each boost is a one-time purchase.