There's been a lot of speculation (that I don't think we've discussed much here) that Spielberg/Lucas are using the new movie to help pass the torch to Shia LeBeouf so that he can become the "new Indiana Jones." If that happens, and there are other movies in the works, then I could very easily see this line continued on for the next several years in support of those new films.
If that's bogus, or doesn't pan out for whatever reason, and the movies end with Crystal Skull, then yeah, I don't see it lasting very long. Maybe mid-to-late 2009 or early 2010. I'd be fine with that, too--I'm pretty happy with what we've seen and know about so far. There's just a couple more figures I'd really like to see, and something tells me they're probably in the works already.
That said, Hasbro seems to be fully backing this line--maybe they know more about the future of the Indy franchise than they're letting on. (Of course, they have to say they're "preparing for a long run at retail" or whatever. Can't really say "well, we're just planning on a few waves for the new movie and hope for the best until the sales start to head south," can they?)