It sounds like you already know that the drug is unsafe for animals, but to reinforce it, the FDA states, "the Pet Poison Helpline states that the prognosis for cats that ingest the medicine is grave. If you apply fluorouracil cream or solution to your skin and you touch your cat or your cat touches you, she may accidentally get some of the medicine on her fur. When she grooms herself, she could ingest the medicine and potentially become sick and die." I don't think you could pick a time after which it's safe for your cat to lick your face with the medicine on it, because there are too many variables. I'm a dog owner so I only know what I would do regarding my dog in a situation like this, but maybe it would apply to cats. I would ask my vet to give me tips for stopping the behavior and maybe do a little training. By the way here is the link to the FDA article: https://www.fda.gov/animal-veterinary/animal-health-literacy/dont-expose-pets-prescription-topical-fluorouracil-medicine-people
I found a lot of stories about why cats lick humans and how to make the stop. Some suggestions wouldn't apply to your face (like put on long sleeved clothes) but others do, so if you take a look, hopefully you can get some ideas. https://www.petmd.com/cat/behavior/evr_ct_why-does-my-cat-lick-me
Good luck and don't be deterred and stop using the Efudex. It could save you more trouble down the line. Sincerely, Ronni (Skincancer.net Team Member