maybe 10-15 minutes at most the first time you do it, then 5 minutes.
There's a plate that installs under the seat, if you're using the stock seat you can leave the plate on and barely see it.
There are 4 bolts (included) that fasten the plate under the seat to the frame. Pull your seat, you'll see the threaded holes at the rear of the frame.
The backrest mounts (on each side) to the plate and shock bolt. Very simple, very sturdy.