Probably extremely frequently since the GFW blocks a huge number of random things including even GitHub once in a while.

Clearly the party was censoring GitHub because of a couple particular repos, it would just muddy the picture if you said that github counts. Not to mention that most of the people in my family and most of my close friends don't visit github.

Good point - I certainly uses GitHub all the time and I should thank you for pointing out my bias. Most of the people probably won't care, but then again, aside from Google, most people here probably won't care about the censored websites either. It's also worth mentioning that Google's Chinese search is atrocious although that might have happened after they quit China.