I am a 40 year old female and just found out I am slightly anemic. How long should I take an iron supplement?

Iron deficient? Question #1 - are you iron deficient? If not then iron pills will not solve anything. The best blood test for iron level is a ferritin test. Other anemia tests include a peripheral blood smear (pbs) and reticulocyte count. There are many other tests that can be done, but that is a start. Anemia may = iron deficient but not always. 1st is figuring out the type of anemia.