Lynn Tsai and I, with the help of others, have been measuring how long it takes for Tor Browser's default bridges to be blocked.
https://arxiv.org/abs/1605.08808 (click "PDF")
Abstract: Censors of the Internet must continually discover and block new circumvention proxy servers. We seek to understand the pace of this process, specifically, the length of the delay between when a proxy becomes potentially discoverable and when the censor blocks it. We measure this delay by testing the reachability of previously unseen Tor bridges, before and after their introduction into Tor Browser, from sites in the U.S., China, and Iran, over a period of five months. We find that China's national firewall blocks these new bridges, but only after a varying delay of between 2 and 18 days, and that blocking occurs only after a user-ready software release, despite bridges being available earlier in source code. While the firewall notices new bridges in Tor Browser, bridges that appear only in Orbot, a version of Tor for mobile devices, remain unblocked. This work highlights the fact that censors can behave in unintuitive ways, which presents difficulties for threat modeling but also opportunities for evasion.
The best summaries are on pages 4 and 5, which show in graphical/tabular form the dates of releases and how long the bridges remained reachable after. We would appreciate any comments or corrections. In particular, the description of the Tor Browser release process could stand some fact-checking by a Tor Browser developer.