Ah, it does not expire.
In fact, DevAddr may be used by different nodes at the same time. The back-end tries the NwkSKeys it has for the concerning DevAddr.
We may want to purge the activated devices after some time, but this expiration time should be long. Afaik there's no expiration specified in LoRaWAN 1.0. There are many use cases possible where activated devices only send a couple of messages per year.