This depends largely on your current fitness level and how high of a cardio work capacity you currently have. It also depends on how fast you run. Beginners will need to gradually increase their weekly training volume from about five hours to around 10 to 12 hours by the end of their training plan.  Very conditioned individuals who have run marathons already can probably get away with fewer training hours, since they are simply maintaining their fitness and not establishing a new cardiovascular base.