Some (e.g. a commenter on one of my May 12 posts) think the Dow couldn't possibly fall 50%, but there is no objective support level for a falling market, it's just a balance between buyers and sellers.
The Wall Street Crash started dramatically, but took about 3 years to complete its decline. On September 3, 1929 the Dow reached a peak at 381.17; by July 8, 1932 it stood at 41.22.
Serious, experienced analysts like Michael Panzner and Peter Schiff tell us that in some respects, the systemic financial problems we now face are indeed comparable to those times. Our advantage is that we have that history to warn us.
More recently, the FTSE 100 reached a peak of 6,930 on December 31, 1999 and a low of 3,287 in March 2003, as shown here. That's a drop of over 52%.
At the time of writing (5:42 pm GMT) it stands at 6,649, but you have to see this in the context of massive monetary inflation; compared with the end of 1999 in real terms, it's lost a lot of ground already. That's why I've suggested that we may already be in a bear market that is disguised by inflation. As in Alice in Wonderland, it has to run quite hard just to stay in the same place.