when and how often should you change up your workouts?

The only time I suggest changing up your workout is if progression stops and/or  you get bored.

     

The only way you are going to make your body change is gradual progression. A lot of magazines and fitness tip websites will tell you to change up your workout program every X amount of days or weeks to "shock" your body ( 6 weeks, 4 weeks, 2 weeks, etc.) 

      

Ask your self this question.  How can you possibly make progress for your body if your constantly changing your workouts? For example, if you are lifting 80 lbs for 9 reps the only way you are going to make progress is if you try and push yourself to 10 reps or going to 85 lbs for 9 reps.  If you are jogging for 8 min before having to stop.  The only way you will get results is if you  jog faster or gradually increase your time.  Example: increase 20 seconds every time you jog.

     

If you are bored with your workouts try super setting: working opposing muscle groups without rest. For example, if you are training chest and back, do one set of chest and go right into the back exercise then go right back to chest and so forth. Super setting is a great way to make progress in you workouts.  If you are jogging on a treadmill for cardio you can try going outside or riding your bike for a change. 

  

The point I'm trying to make is don't change up your workouts just for the sake of change. Stay consistent, have fun, and push yourself!