As the achievement says. all you need to do is just play for 30 hours. I personally did this by 100% (Well all of the available missions as of now) the game. The game does have about 6-28 hours of content depending on how well you do it or if you just rush through the entire missions just to get to the ending and say "I beat it." if you just rush through it you're looking at about 7-12 hours (More or less depending on the difficulty) of content. If you add in all of the properties, cribs, challenges, missions, Genki Gameshow games (Yes they are hilarious if you ignored them) then you're tacking on about 10-16 hours of extra game play (Granted on what difficulty you run it on and how many times it takes for you to do them) Now i just got it today and i only had the game for about 5 days so it's not that hard to rack up the hours when you're lost in the city of steel port.
Joined character files (If you make 15 characters with 2 hours each on them) You will unlock the achievement. It is for 30 consecutive hours of game play for ALL files not just one. So good luck and have fun guys :D