Quite honestly, some rabbits will just use a litter box. Now it depends if it will have the run of the house, or if the rabbits will mostly be in a cage. When litter training your bunny, always reward them when you see them on the litter box, whether or not they're actually using it. If your rabbit poops anywhere else in their cage, move the poop into the litter box, instead of just cleaning it up. This shows them that THIS is where in supposed to go to the bathroom. Then, clean up the area where there went really well. Another thing you can do is, whenever you see your rabbits assume the position that generally means- I'm about to go- move them onto the litter box.
Time wise, it really depends on the rabbit. If you are consistent in your training, it shouldn't take that long. My rabbits got it in a few weeks. Good luck!