So the 25 month (or whatever: http://en.wikipedia.org/wiki/United_States_twenty-dollar_bill#cite_note-2 is a dead link) statement seems to come from the people who _make_ the bills (Bureau of Printing and Engraving). We all think that what we do is more important than it is, so that's not too surprising. They're going to want to pull those little puppies often, if only to maximize their job security. Then again, maybe before 2007 cash money wore out faster because people used it more. A lot of things have changed over the last few years.
The Federal Reserve, by contrast, thinks they last a lot longer (7.7 years). And given that they're the ones who actually pull the duds, in practice, they win.
This not-very-good source (pearson/information please/fact monster, claiming they got the info from Bureau of Printing and Engraving, again) thinks $20 bills last about 4 years:
I'm going with what the Federal Reserve says, because they are the ones functionally deciding what to pull out.