A number of people have noted it can take quite a while, for others it has taken only 10 or 15 minutes, if its still progressing I'd be inclined to leave it alone, however if you are sure it's stuck, you can't do much else besides pull the plug. If it causes further problems you should be able to restore as below:
Get yourself a micro USB cable (sold separately), you can restore your Apple TV from iTunes:
1. Remove ALL cables from Apple TV. (if you don't you will not see Apple TV in the iTunes Source list)
2. Connect the micro USB cable to the Apple TV and to your computer.
3. Reconnect the power cable (only for Apple TV 3)
4. Open iTunes.
5. Select your Apple TV in the Devices list, and then click Restore.
On the other hand you may be able to simply start the update again. If however the update continues to fail, you should try the restore routine above.
If it is the latest update (5.2) and you are connected by ethernet rather than WiFi, you need to update 5.2 via WiFi. All the updates work fine using the ethernet up until 5.2, which can be accomplished using Wifi. Once complete, you may re-establish ethernet for your connection. Or, you may also use WInston Churchill's solution, but mine is faster and does not require the micro USB cable.
This is not quite correct. The wifi/ethernet issue existed when updating from v5.1, not when updating to 5.2, the subtle difference is that there was 5.1.1 in between. In most cases users will now be updating from 5.1.1 to 5.2 and won't encounter the wifi/ethernet problem. Of the few that update straight from 5.1 to 5.2, it should be noted that the wifi/ethernet issue only occurs in a minority of cases dependent on which router the user uses, it should also be noted that the vast majority of users use wifi anyway.
As such in a very small number of cases, a user may benefit from trying wifi first, but the occasions that this is now helpful is so few I no longer feel it worth mentioning.