Whenever you feel like everything is getting slow and progression is halted you can then start to change things up. No number of weeks can determine when to change a rountine/program. It's dealt around many factors. Mainly just who you are since everyone is different.
Changing your workout routine comes with time. If you continue to lift for a couple of years straight you'll get into that groove and know when the time is right to change things up.
Just focus on lifting hard and concentrate on working the muscle you're working. You can have the best program in the world, but can have the sloppiest ways of using it.
Good luck & have fun!