I hear this question a lot from new lifters and, like you might expect, the answer isn't straightforward.  Though there is some common ground between the different methodologies to lifting routines and body building, each one has its own belief about how much rest is necessary.  This article is a brief overview of each of these methods.

The Body Building Method
Body builders swear by working out six days a week, targeting different muscle groups with each workout.  One day they will work chest, the next could be back and shoulders, and the next could be legs.  They do heavy isolation lifts designed to work each muscle group as rigorously as possible.  Each workout is composed of several movements or lifts, each designed to hit the target muscle group in a slightly different way. Because they are working the same muscles during a workout session, training can take upwards of an hour per day, due to the amount of rest needed between sets and movements.  They only take one day per week off for rest, though each muscle group essentially gets worked once per week.

The Compound Method
The compound method involves working out only 3 to 4 times per week.  The workouts are not segregated into specific muscles, but they are grouped by movement.  The idea here is that, while your body as a whole gets more rest, the different compound lifts stimulate your muscles in dramatically different ways and help them grow bigger and stronger in the least amount of time possible. Exercises in the Compound Method include the bench press, deadlift, squat, overhead press, and power clean.  Three of these exercises are performed once every other day, three days per week (ex: Monday, Wednesday, Friday).  The extra days of rest give your body the time it needs to rebuild your muscles and fully recover between workouts.

Which Program is Right For You?
Both programs are effective and you should choose based on the amount of time you have to spend at the gym and whether your long-term goals are to simply look good or to build function strength.

Regardless of which program you choose, you want to have a healthy diet full of protein, calories, fruits, and vegetables.  Eating lots of food will help your body recover faster and will give it the building blocks it needs to rebuild your muscles.  You may also want to consider a nitric oxide supplement to augment your body's natural nitric oxide production.  Nitric oxide is responsible for increasing blood flow to your muscles which helps you lift more weight, recover from your workouts more quickly, and build muscle faster.  It is one of the most studied supplements in the world, with over 70,000 peer-reviewed articles published to date.