Your hair should be 3-5 inches long to reattach dreadlocks. For thinner/skinnier dreadlocks 3 inches is sufficient in most cases. For thicker/fatter dreads 5 inches is ideal.
If attaching to the end of a pre-existing dreadlock we comb out enough hair to create the attachment. As long as your hair is relatively strong and healthy this can be done without much of a problem.