I got approval from AT&T a couple of weeks ago (only took two days after filing it online). I called to make sure there was no timeframe and they confirmed that there wasn't.
Interesting side note: I originally had an iPhone 6 on my line as an upgrade. When I decided to give it to my wife and buy a 6 Plus, Best Buy said their policy was that it had to be activated before I left the store. So, I had the new phone (and sim) activated on my wife's line and then when I got home, I put the new activated some in her iPhone 6 and put my original sim into the new Plus.
Well, when I got the email confirmation approval from AT&T for the unlock, it came with instructions to achieve the unlock. One of the steps was that the original sim purchased with the phone had to be in the phone for the unlock to work. My wife was out of town, so I couldn't get that sim, and that didn't sound right to me anyway, so I just did a restore in iTunes, and the phone unlocked anyway! So, AT&T doesn't know what they are talking about in that regard.