I Have Been Using Fluorouracil Topical 5 Since 11/23. Doctors

I have been using Fluorouracil Topical 5 since 11/23. Doctors instructions were to use it for 28 days twice daily. I contacted the Doctor's office today because I am due to return to work tomorrow on my regular schedule. I am having pain from the treatment and the crusted inflamed skin and requested that she provide a doctor's slip so that I could miss 4 days of work. I work in a power plant and regularly climb ladders, shut or open valves etc and the movement of my back (the treated area) is very painful. The nurse returned my call and said that if it is that painful just discontinue use. I have gone through 21 days of discomfort and am concerned. Will 21 days take care of the skin cancer? I certainly don't want to have start again. What is the normal length of treatment.