Based on my experiences with the tech crash, 2008-2009, and the correction in 2011:
(Assuming I'm still working)
10% drop or bigger:
1. Switch to buying all VTIAX rather than splitting with bonds like I do now.
2. Try to ignore my plummeting total at VG.
3. Let myself freak out, but don't act on it.
4. Keep working and pout/moan/wail/pull-my-hair over the fact that my FI date got pushed out.
For big drops like 50%:
5. Worry some about the state of the world and if Really Bad Things are coming.
I'm currently 1.5 years from FI and 3 years from ER, so any big sustained drop could absolutely have a major effect on the amount of time I need to keep working.
The 2008-2009 crash was scary. Really effin' scary. I won't deny that another one like that would have a major, negative, emotional effect on me. However, I managed to stay the course despite that, and I expect I'll do so again in the future.