Generally, nails should be short enough that they don’t touch the ground when the animal is standing/walking normally. Like Megan said, frequent trimming will shorten the quick over time. I just ran into the same problem with our dog, the quick seems to have grown a bit too long on his front paws. Another method of training back the quick is to cut the nail as short as you can without nicking the quick, and then carefully shaving off the nail around the tip of the quick on the top and sides, so that the quick ends up sticking out somewhat exposed. The quick will then shrivel up and fall off without causing any bleeding, and next time you can cut the nail a bit shorter. You can repeat this process until the desired length is achieved. Doing this without cutting the quick is of course pretty tricky, and since rabbits have such small nails, it might not be a good option. Using a dremel makes it a little easier/safer, but most rabbits would be scared of the noise.