In a recent interview, Harold Perrineau from the cast of Lost stated that J.J. Abrams and Damon Lindelof (writers/producers of the show) have mentioned that they have an eight year story arc in mind.
I'm sorry, but I don't think that I could stand to be teased that long before finding the answer to the big mystery that ties everything together. Heck, I groaned when I found out it was going to be picked up for another season, much less 5-7 seasons more after that. (BTW, there is no word that the series will actually go on that long for sure)
Chris Carter and X-Files showed that you can only string people along and yank their chains for so long before they finally give up on a show and just say: "Whatever".
I don't know... I think the angle used by the writers and producers of 24 is much more sustainable: Taking essentially the same characters and every season putting them in a different situation. Even that has the possibility of getting old, but the 24 team are extremely adept at throwing in twists, turns and cliffhangers that keep it (for me, anyway) fresh and exciting.
I always thought the concept of Lost was well-suited to a one season or maybe two season run, but that's it.
Thanks to Dark Horizons for the heads-up.