To get this achievement, you will need to play multiplayer on this game for a grand total of 80 Hours. It can be done across all game modes, as the time is cumulative, meaning you don't have to play just one game mode alone, to get it.
There are two ways in which you can do this achievement. You can either leave your console on and just keep the game running. This would be 3 Days and just over 8 Hours.
Or, if you're like me, and you don't fully trust your 360 being on for extensive periods of time, then an alternative would be to just leave it on for certain periods of time.
Before I start, may I suggest two additional pointers when you are doing this achievement.
Firstly, I would suggest you save this as your "last multiplayer" achievement in the game. The reason being is because the other achievements will require a bit of time in order for you to unlock, and by the time you have done all of them, you will put in at least 3 - 10 Hours work, which will go towards this achievement as well.
Secondly, if you have a Physical Copy of this game, it is recommended to install the game to your Hard-Drive. This will prevent the console from overheating, and will decrease the loudness of the console, since it will be running from the Hard-Drive. It takes approximately 4.6 GB to install.
OK, I have provided a step-by-step guide into helping you create a private match, by yourself, which I used into getting the achievement for myself. You must do this the first time you set this up, but the game will save it, until the user decides to change the settings. This means, that the next time you whip on the game, you can actually just create a match quickly, with the settings already in place...1. From the Main Menu, select Multiplayer, and then click Player Match
2. Click Create Match
3. Game Type: Chaos (Can be anything of your choice)
4. Score To Win: Unlimited
5. Map: Any (A small map is recommended, I originally used Sunset Bar, but then I decided to use my Custom Map)
6. Time Limit: 45 Minutes
7. Leader Icon: Yes
8. Press Right to scroll to the next page
9. Host Type: Normal Host
10. Join In Progress: No
11. Maximum Players: 2
12. Private Slots: 2
13. Auto-Launch Time: 15 Seconds
14. Voting: No
Once you have done all these settings, click Create Match. You will now be taken to a lobby. The Auto-Launch will start a countdown of 15 seconds. You can actually launch the game immediately by pressing the
Button, this will say "Game Starts" and a 10 second timer.
You should now be in the game, with yourself only on the map. You can now put your controller to a side, and let the game do the work. Unless you give out an invite to somebody, no one should join your game.
When the match is done, the game will automatically take you back to the lobby. It will then do a 15 second Auto-Launch timer, and a 10 second Game-Start timer. After that, the match with the same settings will reload.
I have also discovered that you "can" actually host the same game modes you want, with just yourself in it, under a Ranked Match. There is however, advantages and disadvantages though. The advantage would be that you can actually check the leaderboards in Ranked Game Modes, and find out how much "Time" you've exactly put in towards the achievement. But the disadvantage would be that if you get Disconnected from XBox Live, you would return to the main menu, rather than the matching continuing, which means you could lose progress, if you got disconnected during your sleep.
On the other hand, if you decide to do this achievement in Player Matches, if you get kicked of XBox Live, the match will still continue as when I was doing it, I got kicked off XBox Live on a few occasions, and the match still continued, which means the time for this achievement, will still continue, regardless of the connectivity.
Hope all of this helps. Good Luck!