Keep them forever.  It's easy these days scan and convert files to pdf format to store on your hard drive.  Our office has a very fancy machine that does it in no time.  For my home office, I have a HP Laserjet 3015 that works nicely.  It also faxes, copys and does the dishes.  I'm pretty sure it was only around $300.00 or so.  Maybe less.  Toner cartridges are around $75.00 and last a long time.  And drive space ain't so expensive.  Cheaper than the warehouse you'll eventually need to store all of the paper.