Since 1997 to be precise (or atleast that's the first official announcement of the project, also mentioned in the 'Shadow Warrior' game) which means 8 or maybe even 9 years. But they've changed 3 engines since they started (Doom, Quake and now I think they work on their own engine) so don't expect it to be the perfect game. But I'm still looking forward for it.
And, yes, this job sure is sweet. But as they say, you have the be one of the best. Someone like Firebinder and Manoid or the like (Lost Time, Xen Pit; both from Issues).