How long does it take for your kittens to not be contagious once treatment for worms has started?

I will assume this is in reference to a canine. Generally, depending on type of worm, a day or two is usually the length of period the dog stays lethargic and should be left to rest. During this time the worms are being murdered and are or will be leaving the host's body either through regurgitation or excrement. This does not apply toward heartworms and the vet will consult you on their behavior when receiving this treatment.