...You would be surprised at the number of people who don't react right away. It's definitley not unheard of to go for a couple of weeks without seeing a big reaction. There's always the chance you don't have as much as damage as you and your doctor initially thought. Reactions really vary from patient to patient.
Efudex is a real mystery. Some react quickly, others barely get more than slight irritation. Give it another week. If you are using it twice a day and applying a thin layer, you are following the prescribed treatment. I would guess that if you are going to have a reaction, you will begin to notice it in the next week. I have even noticed that I vary with reaction times from one treatment to the next. One time I might react in the first few days, other times it might take a couple of weeks.
After the 28 days, check back in with your doctor and see if they advise you to continue or stop treatment. You may be asked to drop in or send pics of the treated area.
I am including a couple of articles that might come in handy as you move forward.
https://skincancer.net/life-with-skin-cancer/tips-for-efudex-users
https://skincancer.net/life-with-skin-cancer/efudex-cream-how-long
Wishing you all the best! Don't hesitate to reach out! April, SkinCancer.net, Moderator