Quote validity is a single setting:

1. Go to Settings → "Facturen en offertes"
2. Under "Hoe lang blijft een offerte geldig?" (how long does a quote stay valid?), pick 2 weeks, 1 month, or 2 months
3. Changes save automatically

New quotes get their valid-until date from this choice. Once that date passes, the quote shows as Expired in your quote list.

Tip: The Expired status appears automatically, you do not have to update quotes yourself.