Hey, I'm still using the iPad 1, which I bought the first week it came out. I don't really care if I have Siri on it or not, the things I do with it (Pages, various photo software, music, Youtube, reading the newspaper) still work fine.
So they last quite well. I think physically the limiting factor is probably the battery, mine is still going strong, YMMV. The other limiting factor would be whether you'll need whatever additional functionality that comes out 2-3 years down the road and whether the iPad you buy now will still be compatible with it.