I've just started strength training again, and I'm wondering how to know when it's time to switch up my routine?

I've heard that once we can easily do 8-12 reps (I'm assuming that's on the 2nd or 3rd set), it's time to transition to a heavier weight. But how often should I be changing the exercise I do? And how do I know which exercise to replace it with? Same goes for cardio, I guess. How do you know when it's time to change things up?

I think 6-8 weeks is the usual for a new routine, but I'd keep the bigs in there and just mix up the rep scheme: deadlift, back squat, etc. Sorry, been doing Crossfit for a few years, and we rarely do the same workout twice, so it's hard to remember!

It's 4-6 weeks... I am now doing Group Power classes and she changes up every 6-8 weeks, by the time the 4 or 5th week comes along I am ready for a new routine, instead I increase my weights and that helps a lot... When I was weight training on my own in the gym I would change every 2 weeks because I get bored easily... But rule of thumb is 4-6 weeks...

You usually replace it with an exercise of the same muscle group... For example squats, you could replace, or add to your routine, Sumo squats... Here is a great site http://www.exrx.net/ for losts of varitions of exercises...

As for the cardio I change my routine up by adding intervals ie: HIIT, or hills, duration, etc...

For myself, I know when to change things up when I am BORED...

Hope this helps....

Thanks!

When you get tired of it?