FDR was in office for over 12 years and didn't. Truman didn't [served nearly 8 years].
DOW Jones graph:
http://finance.yahoo.com/q/bc?s=%5EDJI&t=my

And length of time held office of US presidents:
http://en.wikipedia.org/wiki/List_of_United_States_Presidents_by_time_in_office

And of course W Bush hasn't yet served 6 years [I know some think it's been forever:]

There aren't very many presidents that have served for 6 or more years- if you mean in 20th Century and into the 21 Century, I would say roughly that less than half didn't and ones that didn't, happenned to be Dems. Clinton of course did serve during a time of long bull market and if you didn't limit it to 6 years, Hoover [A republican] was in office during The Stock Market Crash, though the Republican Coolidge was in office during the Roaring Twenties and was in office for more than 6 years [in the two terms before the The Crash].
Looking back in history, W Bush will have regarded as fairly lucky to have gained and held the presidency for 2 two terms. His success at gaining office was largely due the Democratic party being incompetent and particularly incompetent at selecting their leadership [e.g. Gore and Kerry] and in general their inability to provide real choices in terms of policy.