If your doing it on jackstands in your driveway like I did, and its your first time... plan on a few hours. Its not hard, just annoying and difficult working under a car thats not too high off the ground.
If you access to a lift and all the tools, an hour, maybe.
Thats if everything isnt rusted on of course, like svttech mentioned.
And for the UPR (not sure about others) you put it together in two peices. The drivers side tubing that mates up with the manifold is seperate from the rest of the pipe. You wont get it up in one peice, unless your magic or something.
Also, my manifold flange gasket blew, so when I started it back up it sounded like ass and had a huge leak... so if you have a chance grab a flange gasket just in case, they are 5 bucks or so.