In the best case scenario, I think exercise should regularly include all major body parts. Fortunately many types of exercise, such as running, already do this. However weight training and other related activities usually require more effort and planning.

For strength training, it's generally recommended you workout each muscle group at least once a week. Otherwise, as any body builder would tell you, you could develop your muscles (and muscle groups) unevenly. Exercising some body parts more often than others could also lead to injuries.

While you want to target every body part, you need to give each muscle group time to recover, and some take longer than others. This largely depends on the type of exercise and body part. For example some areas, such as your abdomen, can be exercised several times a week and some exercises (like pushups) can even be done several times a day. However most people would wait a few days between chest workouts involving weights (like chest press).