Do stamps expire?
Only if the place has set an expiry. You can see this on the card.
By default, stamps don’t expire. Each place decides for itself whether to set an expiry (for example 12 months). If it does, you’ll see it in the card details and in the place’s rules.
If the place has turned on the reminder before expiry, your card lets you know before the stamps expire: on the card page, and as a notification too if you have turned notifications on.
The reward is always given by the place, and the place sets its conditions. The reward can’t be exchanged for cash.