I was probably anemic most of my life before starting an iron supplement (pills, not liquid. I hate liquid) the summer before my senior year of high school. My PRs up to my junior year of high school were:
800- 2:40
1600- 5:39
3200- 12:21
5k- 19:30 (road race)
My senior year of high school I ran:
800- 2:19
1600- 5:04
3200- 10:39
5k- 17:46 (road race)
It was absolutely fantastic. I even had charges of EPO usage leveled at me, which wasn't so fantastic but still flattering in its way.
I'd say it took 6 months for my iron levels to become normal. Throughout cross country I improved by 30 second leaps about every two weeks before becoming consistent during track season.
Not everyone will respond this way to iron supplementation. I like to believe that training for a long time with anemia gave me greater training benefits that showed up big time when the anemia went away, but I have nothing scientific to back that up with. Also, my levels were significantly low. The only number I remember is a 29 hematocrit with normal being 36-41, I think.
I still take an iron pill every day. I either don't eat enough red meat or my body has poor iron conservation methods. Or both.