Eric (from Instructables) has had his sweater for seven years... according to my notes he washes it about once a year (this is useful so we know how much he's saving resources-wise vs turning on the heat). In his words:
This sweater is one I keep at work. It's always there in case I've underdressed and the office is too cold. It was way too big when I bought it and I had to run two seams up along the sides so it would fit me better. It still hangs a little funny on me, but it's plenty warm.