How much Lysine is safe to give to your cat as a maintenance dosage?

I adopted a kitty a year ago with feline herpes. Under the vet's recommendation, I put her on 500 mg of Lysine twice a day. The vet said it was okay to keep her on it indefinitely, although probably not necessary. I have also been giving that dosage to my two other cats, in the hopes of protecting them from the herpes virus. (The kitty who was sick has since recovered.) Now I'm wondering whether to reduce all of their dosages to a "maintenance" dosage. On the web, I see some recommendations of a 500 mg once a day maintenance dosage and others say 250 mg once a day. But my vet had said that if you don't give 500 twice a day, there's no effect. Has anyone had experience with keeping cats on any kind of maintenance dosage of Lysine?