It will take no more than 20 - 25 mins first time you do it.
Was well easy, follow the manual, but I took the chain off first then the rear wheel. Slacken the three bolts on the swing arm, undo the pivot bolt and withdraw it, move the swing arm back out of the way and then you can get at the two bolts that hold the sprocket on.
Replace in the reverse order.
Makes the bike much easier to ride off-road with smaller gear on the front.