I cut myself shaving my leg and its really red- how long does it take for redness to go away- this was a week ago?

May be infected. If yu have an open wound that is getting more painful, with spreading redness, increasing skin temp, or drainage from the skin you may have an infection. Red streaks from the area or symptoms like pain behind the knee or at the groin area indicate spreading infection, and progressive symptoms like fever, nausea or abdominal painty indicate blood poisoning. See a doctor right away to be evaluated.
Get it looked at. You could have an infection and it will take much longer unless it's treated. No way to tell without examining it. See your doc for an exam if it doesn't get better.
Cut. Redness could signify infection. Any pain, pus or fever? If so, you need to see your doctor. If not, it is probably healing ok.