Personally, I think Orange Box will last much longer than just CS:S. Even though only one of the games is multiplayer online, TF2 will last you a whileeee if you get into it.
Not to mention about 18 hours for Half Life 2, 5 hours for Episode One, 6 hours for Episode 2, and a couple hours for Portal. That's 30 hours of single player, plus however much you end up playing Team Fortress 2.