I played U1 a few years back and finished it in about 4 hours. Probably could have done it faster, but I am one of those that must explore every single inch of the world and complete every quest! I image U2 and U3 are probably the same time frames.
I've never really played U1 or U2 other than to sample them, but I recall U3 taking *much* longer than that to finish. Of course that was a long time ago, and I was pretty young then..
Looks like GOG has Ultima Underworld 1 & 2 as well. Is that new? I don't remember see them there before.
They were released in June.