Don't rely on what your competition does, or what people tell you, or what even your users tell you.
I personally have a very simple rule. I think the best moment to redesign a website (a site, a blog, an app,...) is when you wake up in the morning, you're looking at your current site and you don't like it anymore, or worse. If you can't look at it anymore or don t even enjoy using it for yourself then move on.
If you feel your product and your customers, it will come from the guts. Save your market studies and deep focus groups. If you own a site, the best to grow a website to the next stage is when you feel the cloths do not fit any more.
This is what i have been feeling for a while with AppsFire..