I believe we'll keep going strong until at least the end of the live action TV series - which if it starts in 2009, should run until 2013 given the episode count they're talking about - perhaps a year after that we'll see it finally slow down in popularity unless there's something else coming along to keep the interest up - another movie, another TV series, whatever - and given the Lucas empire, I strongly doubt that they'll let it go.
As far as figures are concerned, I think the line will continue to move forward very well. Seeing as we're now effectively two years after ROTS, sales and release quantities seem very similar, and the interest seems just as strong. The same could not be said in 1985 when Star Wars collecting previously "died."
What will the figures be? I think they'll continue with the format for this year (obviously with tweaks and the requisite packaging changes), introducing a stronger dose of TV-based figures over the next couple of years as those productions ramp up. Considering some of the wish lists that I've seen (my own included) I know there's plenty of remaining movie-character options - my current list is around 250 - so there's more than a third of your "600" figures. There will always be some minor variation they can do on the main characters to keep those in the line (DS Escape Luke and Han - take the VOTC figures and add Stormtrooper belts or Lando "Ewok Celebration" - for a couple of examples) as well as PUH-LENTY of Cantina Denizens, Jabba Henchmen, Ewoks and random soldiers.
I do believe that at some point they'll move to a true "collector" series type of release where the figures get so obscure, or so long-term collector focused, that they'll have to come out separately from the main line, and likely at a higher price due to lower distribution. Again, for example, I really want ALL of the Padme outfits that they haven't done yet, and there are something like 25 of them - it's doubtful that ALL of them will make it into the basic line. So if I have to pay extra to get the "Kohun Attack" Padme because it's in a special collector focused line, so be it.
There will also continue to be a very healthy dose of video game and comic book characters for the EU segment - FU and similar releases will continue to garner more and more attention with collectors - Darth Revan is only the start this year, with any luck he'll be as popular as the polls indicate he will - as that just shows that the EU segment really does have a market and that they can grow it. Many of you from that "other" board know me as EU hater, but that's only when EU is chosen in favor of movie characters - at this point I look at it a little differently, that the EU is one of the keys that will keep the interest in the line over the next several years, which may keep the movie characters coming - so long as there's interest, Hasbro will keep making them.
I also question the 600 estimate - I think it will be higher by the time you count two-packs, battle-packs, collector multi-packs and similar non-basic releases - so let's bump that to 1000 figures between now and 2018 (IF it continues that long) - I'd guess that it'll break down into something about like:
350 TV & "New Cannon"
300 EU - VG, Comics, Novels
Sure there will be rehashes and re-releases in that mix, but over that 10 year run, there will be a LOT of great stuff.
Remember that 2017 (40th Anniv. to the math challenged) is the year before that "end" and lord knows what Lucas will do around that time, something tells me that somewhere in the back of his mind, is the desire to "remake" Episodes 4, 5 and 6 to make them "fit" better with the PT. An unholy comment to most here, but he's the kind that constantly meddles, so you can't rule out that sort of possibility.
Then we get to start ALL over again!