The human body is fairly adaptable, but it also has a natural desire to maintain the status quo. Meaning that, when it comes to exercise, your work is never really done because you need to keep challenging and surprising your body. As this is true in cardio, it is true in strength training too.
You should aim to change your weight training program every four to six weeks to prevent the plateau period that comes when your body adapts to stress. You can either increase the weight or the number of reps, but you have got to change it up.
Fit's Tip: To keep your routine interesting during that 6 week period, up the weight on dumbbell and medicine ball exercises and do half as many reps. Try that every 4 to 6 workouts.