My cat has ringworm, how long is it contagious after treatment?

I hate to hear that Boo has ringworm. The main problem with spreading is due to the spores of the fungus found in the environment. The spores are attached to pieces of shedding hair and can be active in the environment for months in ideal conditions. Environmental clean up is a must to control this fungus.
As far as Boo himself goes, the topical treatments will help to remove the contagious spores faster and the oral medications will help to kill the fungus faster which will result in less spores. Unfortunately, Boo is considered contagious until he has two negative cultures done 1 week apart. Most cats will be start re-testing for ringworm 2-4 weeks after initial diagnosis. Here is a very helpful link about ringworm including environmental clean up:
https://www.petcoach.co/article/ringworm-in-cats/

I hope this helps and please feel free to post any additional questions.