I finished the SP game in 20 hours on Jedi Knight difficulty, playing the Light Side. That was the first time through, and the longest I played it. Using the Dark Side powers makes it easier on the same difficulty level, and because I knew where I was going the second time through, it took a lot less time to complete - probably about 12-14 hours.
Since I beat it twice, I've been playing MP...mostly Siege for the last few days. They've done a great job with the Siege mode - all it needs are a few little tweaks and some extra maps.
As others have said, SP has some replay value, just by messing around with the options available, and MP looks like long-term fun.