I watched Redjake play MGS3, and trust me, it's the longest of the series. Most reviewers are saying it's 20-25 hours long the first time through but he beat it in about 17 hours. 17 hours!!! The first MGS took about 6-10 hours the first time through and MGS2 wasn't much different. He did beat it in 2 days, but he didn't stop playing it either.
It's worth the money. It's certainly not "easy" either!
-Lasereth