A few hours. I would probably have beaten the game, but I have to play it on a Emulator since PS2 look awful in my TV (I have been looking for a nice CRT TV for ages). And I have FPs problems on the Emulator.
I need to try it sometime. But considering how much I like complaining about stuff I don't like I'd probably play the whole thing, twice, then come back here and preach how it's actually not that bad and defend it alongside Fallout 3.
I beat it, had too because of the Marathon I'm doing. It was super repetitive and unrewarding, however at the same time I did enjoy it (to a certain extent). I would recommend you play it once, just so that you can moan about it and then say, "Oh yeah I beat that piece of shit".