I usually keep mine until after tax time the following year since I usually have a good number of conferences and work-related trips to write off.
This means that in actuality they have a tendancy to hang around a couple years. The really odd thing is that my brain is so tuned to trivia that I can usually remember the trip by looking at the BP, even if it's something I do a lot like a transcon back east or Sh*ttle up to SEA or LAX.

JD