Easiest way to do this would be to start a flight that goes over the ocean, engage autopilot, then let it run for a few hours. I used to decrease sim rate (because it counts real-time so it makes the flight safer/longer) but some people are saying that sim-rate is making the game not count time during the flight. Unlimited fuel should still work as well, don't forget to enable that in the assistance menu.
The following guide is currently not working and was potentially patched:
This achievement (and other similar achievements, ie: 50 hours at night/IFR/rain) are considerably the easiest and fastest achievements in the game.
To speed these up, simply start a flight in whatever conditions you wish, fly for a couple minutes, pause the game, then tab out and change your computers date and time to sometime in the future, don't go crazy, just do about a week or less at a time, if you get the message that you lost connection, reset the time and try again with less of a time gap. Then simply go back to flying and watch the achievements role in.