On an ATX, it's about an hour's job. I've found the most time consuming part is removing & reinstalling the battery tray. The three bolts that hold it down are usually covered with corrosion. It's also a good idea to clean off the tray and repaint it while it's out.
The ATX has a spring-loaded automatic belt tensioner, so you don't have to concern yourself about setting the proper tension. All you have to do is put a (17mm?) socket on the tensioner pulley (located below and between the water and power steering pumps) and apply clockwise torque to relieve tension sufficiently to remove the belt from the alt. pulley. Then just swap out the alt., replace the belt by relieving tension the exact same way and fitting the belt on the alt. pulley, reinstall the tray and batty, and voila!
BTW, before starting/driving the car with the new alt., make sure that you fully charge the battery first. A bad battery can ruin the voltage regulator in an alternator (even a brand new one) in short order.
