I honestly can't remember, but I'm thinking maybe 10-12 months? It's hard to predict, but I think you'll use all of them.
If you were to use, say 8 diapers a day, 400 would last a month and a half. Most likely he or she will be in size 2 for at least that long. I wouldn't stock up ahead too much, though, because babies will sometimes stay in one size (diapers, clothing, shoes) for a while, and then grow out of the next size fast, or vice versa.