The quality of the replacement downpipe and the ease with which it'll mate to the existing system will dictate how easy it is to change.
My stock one was easy to remove - took about 15 minutes the first time I did it, as it's pretty easy to get at the manifold bolts from above. Rear mounted exhausts tend not to be too bad for rusting in place. My last car had 150k on it, the manifold had never been removed and it came undone with no bother at all.
With the benefit of a ramp / 4-poster / pit I'd say 1hour maximum
if it is a simple straight swap.
You'll need ramps with a very shallow angle on them - it's rare for a modern car to have enough clearance to drive on to ramps like these: