I don't remember the exact dates, but at some point I was down about 40% from my peak portfolio level during that 2009 market drop. It wasn't a big deal then, because I was what I considered a long way off from retiring, and the dollar amounts were still amounts that I could imagine earning back via additional investing. So it wasn't hard to stay the course and keep dollar cost averaging through it.
Nowadays, our net worth is 6-7 times what it was at the peak in 2007-2008, and we are only 5-10 years away from potentially retiring. A 40-50% decline in our retirement assets would be feel catastrophic. Staying the course would be too difficult without my 33% bond/cash allocation, IMO.