I got the beta the 1st of december
and in the email it says the expiry date is the first, so you have to download it straight away.
This is to stop people from trading them and giving them away, they only work once aswell!
I think this is good as sony would not be able to create a good game/service if there are more people on the beta than sony planned.
hope this helps,
Oh and the beta is good, gameplay is good and smooth, more content is due when it is out but all in all pretty pleased with it 🙂