Hitting a plateau with your workout

When you do the same type of workout repeatedly for a while, how long does it take before you hit a plateau? I have been very focused on yoga since January when I found www.yoga.com and I have been exclusively doing yoga since then. It doesn't seem that I have hit a plateau yet, but I wonder if that's because I am switching up my yoga routines. In the past, if I did something exclusively for about two weeks, I would stop seeing results.

I will remember your words.