Depends on the bunny, how old she is, and if she's spayed or not. Michaela's right, puberty often sends all littertraining out the window. I have one girl who despite all efforts refuses to be littertrained even after two years.:rollseyes
The others only took a few days, even the two that hadn't seen a litterbox at all for their first year and a half of life. Usually it's a matter of finding out where they want the litterbox, sometimes having multiple, and how they like it. For example, my 3 littertrained permanent buns like having a hay rack above their litterbox. My foster bun doesn't like the hay rack. And if he had one litterbox with hay in it, he's sit outside of it and eat hay while pooping on the floor. So I have to give him two side-by-side litterboxes with hay covering one or both of them and then he has perfect litter habits. It just took some experimentation.