I've always heard that you should not keep an exercise routine for very long because your body will get use to it and it will not be a challenge anymore.

But how long should you keep the routine before switching it out?

2 months?6 months?A year?

Does it vary from person to person?

I'd venture that you should change things up when you stop seeing the intended results from the exercise. It probably does vary widely from person to person and activity to activity.

when the exercises are too easy or you are bored.

The trainer I have at the gym said I should change up my routine every 3 to 4 weeks so that's what I've been doing. I just switched it up this past week so we'll see if it does anything!

Yep every 3-6 weeks it's time to change up... Boredom and lack of change are definitely good indications too go for a change. I change up almost every time I go...