How often you need to work out depends on your fitness level, goals, and time limitations. The loftier the goal, the more time you will have to spend going after it. That said, if you have a very demanding job, a family, and/or other important personal responsibilities, you may not be able to spend hours per week working out, and thus will need to be patient when it comes to reaching your goal. To make decent progress you will certainly need to hit the gym at least 3 days per week for an hour at a time. However, as time passes, you will begin to require more time working out in order to see continued progress. I suggest starting out with three gym sessions per week and adding a bit to your program every month or so.