Normally no more than half an hour, if its still progressing leave it to finish. If not.......
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.