I would go for KotOR-length, with a little more variability for side-quests/exploration. Unless you really "try everything" I'd say 60-65 hours max. Higher than that gets into Final Fantasy level-crunching territory. Sure, there are games that can fill 80+ hours with a truly epic level of content (the Baldur's Gate games maybe), but I would hate for them to change the nature of the series to add length to the game--for me, KotOR is 8-10 planets/levels broken into 4 or 5 maps.
The worst thing possible would be just to just add maps in an effort to increase "playlength"--for example, the first 2/3 of Deus Ex would make the greatest game ever, only they had to add 40 hours of wandering military bases and science facilities shooting mutants and killer robots with absolutely no story to speak of. The player should have a reasonable goal of completing the game before losing interest--not just on the first playthrough, but on every playthrough.