How can I tell when to stop putting fluorouracil cream on?

I am a redhead with very sensitive skin. Before trying the chemo cream on my face, I decided to try it on a less obvious spot, my forearm. I have used it for 12 days and I have about ten red spots of various sizes on my arm, They are itchy and some are starting to feel crusty but no peeling yet. How can I tell when I don't need to put any more on? I see photos of people's faces and arms doing this treatment but my arm doesn't look red all over like their treatments do. There are just isolated red spots. Does this mean it's not done working yet? Thanks for any advice.