Hi Graham
I've been there and done that, time to do is about 10 to 15 minutes for just the basic pad change. It's the same for the front as well if that's all you need do. Just remember to use the old pad and use a screw driver e.g (don't scratch disc) as a lever and gently push the pistons back into the caliper so the new ones fit in.
Your lucky you've got only two pistons per caliper, I've got three
Just make sure the pads locate properly..........or the retaing bolts won't go in.
When it's all back together pump the brake lever a few times. spin the wheel by hand to make sure the brakes work. Then a quick test ride around the block to make sure they are working properly.