You may actually have to dip multiple times to get rid of the problem 100%. A cat is considered contagious as long as you are seeing lesions. Even though the dip will help clear it up, it will not get rid of the problem with a single treatment. You should plan on dipping at least twice a week. Below is a link that will help provide good treatment information for you. Ringworm can be difficult to clear up, but it is possible.
Please let me know if I can help further.