Most of this time will be taken up by playing all 300 matches you have to for the "Forged from steel" achievement. If your not boosting, or aren't super awesome at this game, you may need to play more than 300 matches to get to 100 wins as well, so that's a potential time addition.
Whatever time you may have left, spectate in lobbies. Enter as full a lobby as you can to prevent the need to frequently change lobbies, and hit the X button to spectate. Now you can read a book, watch some TV or whatever while the time slowly adds up.
The time that is counted is ONLY the match time, so sitting idle in a lobby does nothing for you. That being said, check on it every now and again to make sure it's still going. This is crucial if your playing at an odd time when many others may not be.