500 mg daily is the therapeutic (and loading) dose. Once the herpes has been under control for a while (at least six months, but I would say a year) 250 mg a day, or 500 mg every other day is usual for maintenance, with an increase to the therapeutic dose (500 mg a day) for a week or two, when a flare up happens.
Depending on the age of the cat, I would do 500mg a day maintenance and 1000mg during a flare up. If it's a kitten, I'd do 250 mg maintenance and 500mg during flare up. Is this for your little foster girl (Emily??)?
It's for her and for Ella, who suddenly has a weepy eye. Emily is so sweet and loving and playful, but she is wheezy and sneezy, and doesn't seem to be getting better.