I replace mine seasonally.
You get a brand new down comforter every season??
Hmm... I kept mine for a few years before getting a new one. Some people I Know have kept theirs even longer. I always thought with regular cleanings and stuff like that it would last for a lon time. Sometimes they start to lose the down or I had a cheap one in college that shifted a lot, but I was always under the impression if you bought a good one it would last for a long time!