You should workout as many days as it takes to accomplish your goals and have it fit in with your life style. It's a very individual thing.
Some people find Mon, Wed and Fri enough. I believe the 5x5 program is a 3-day a week program.
People doing Westside have 4 workouts a week.
Personally, I try to go 4 times a week, but life's activities sometimes limits me to 3. I've tried 5 and found it too difficult mentally as well as physically. You need time to recover.
You'll be getting many different opinions on this one, so experiment and see what works for you.