Technically, if you didn't use up all the continues, or die at all, and breezed through the bosses, it's probably not even an hour. It's not very likely that anyone would have been able to do that their first time playing the original, though. Most shooters at the time (2D or 3D) were about that long, if you could beat them at all. (In the case of 2D shooters, I really mean 1-credit clears.) The time to get to the point where that was possible was where a lot of the gameplay time would come from. The nice thing about Panzer Dragoon was that the more you started to understand and learn, the better you would do, and the more continues you would earn for the gauntlets at the end of the game.
Zwei adds a bit more to the run-time, but Orta's really the one to offer a more 'substantial' (two-and-a-half-ish hour) experience. I like the original Panzer Dragoon as sort of a time capsule, and an early 3D game done really well, though.
