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....