Europa Universalis III tops the list, with well over 100 hours (Steam's count is a bit off, so I'm thinking around 150), Red Orchestra has about three full days, Guild Wars probably also a day or two at least, some others, probably more from when I was younger. Age of Empires II is the best contestant here.
Interestingly enough, although I have a little under 20 hours logged in CoD4 multiplayer, and over 28 hours in MW2 multiplayer, the time I spent with CoD4 somehow seems longer. At least, more profound, insofar as you can apply that term to a game where the only objective is to shoot everyone.