The last NIE document I got for a client (about 3 weeks ago) clearly states that it expires after 3 months. This was the first time I'd seen this and I didn't notice until I'd left the police station. I will ask next time I am there.

Note that the useless and inconvenient green A4 residency document for citizens of EU countries has been replaced by a new and much handier card (no photo, same size as a credit card). As far as I know, this doesn't expire.

But it smudges very easily so if you get one, get it laminated quick!