According to a couple of blog comments on February 1, 2015, the default obfs4 bridges are blocked by GFW.
https://blog.torproject.org/blog/tor-browser-45a3-released#comment-86416 https://blog.torproject.org/blog/tor-browser-45a3-released#comment-86907
The obfs4 bridges were introduced on November 17, 2014. https://blog.torproject.org/blog/tor-browser-45-alpha-1-released I have a message from Yawning Angel verifying that the obfs4 bridges were reachable on December 2, 2014.
With this information, we can put rough bounds on how long it took the GFW to react to a new circumvention system: between 2 and 10 weeks.
Does anyone know information that could tighten up the bounds, a date between December 2 and February 1 when obfs4 bridges were known to be either accessible or inaccessible?
David Fifield