Depends entirely on the record label. Some are one time download code. but, the files all seem to be DRM free - at least I've never encountered protected downloads. Even with a one time code, you can copy/backup the file as many times as you like once you download it.
One issue I've had is trying to figure out what bit rate the digital downloads are. For example, the new Beck album didn't specify, and I'm worried it will turn out to be 320 mp3. Again, varies, some are very clear about this, others aren't.
I've only encountered a time limit once - I think it was with Paul Simon's Graceland reissue. I bought the album after the code had expired, which was pretty annoying. Customer service also varies a lot by label. In this case, I wasn't able to contact anyone.