If you're efficient going through the story and arcade, you will actually have to leave your xbox on to get the 20-hour achievement. I just finished my last achievement other than the 20-hour and the 2nd playthrough and I'm only at 14 hours total playing time. I wasn't even as efficient as I could have been in story mode either. Since it doesn't carry across save files, I guess I'm leaving my xbox on today while I sleep.
Landed on exactly 500,000/500,000gs with exactly 18500 achievements and exactly 500 games. All 100% including DLC. Was going hard the first half of January to reach that goal! Finished it on 1-19-2016. Final achievement: Completion Serenity in Nero.