We keep boxes until we are sure that we are not returning the item (uh, we do that far too often

) or, like other PPs, if it is for something particularly expensive like our flat screen TV. We used to keep them for moving, which we did almost every year, but even then only kept a few "special" ones since we lacked storage space for much more.
We just moved into our first house, and are still unpacking, but almost all the boxes are going to recycling. For some silly reaon, I did put the flat screen TV box in the basement, though we have no intention of moving anytime soon since this is intended to be our long-term residence. I guess that old habits die hard. We are also keeping the good boxes that we bought, probably to give away to others in need since they are only gently used, sturdy (though not as sturdy as the free apple crates that we got at the supermarket) and I hate the idea of throwing away something that I only just spent good money on

.