The easiest way to get this is through co-operative multiplayer. I received this achievement after 2 nights of playing or approximately 8 matches, and around 5 hours.
If your preference is single player and you start with a fresh playthrough and no Mass Effect 2 import, you will need to do 2 playthroughs with the same character to have a chance of reaching 50. Even if you import a level 30 from Mass Effect 2 it will take approximately 15 hours to get to level 50 so multiplayer is the FASTEST way.