How often should African-American hair be washed?

my hair is relaxed.i feel like i don't wash my hair enough. i'll wash my hair every two weeks most of the time, but since it's summer it can be up to 3 times a week, because of swimming and what not. my scalp get incredibly itchy and i get bad dandruff and my hair starts to smell after a week of not washing my hair ( if i got that long) and i get flakes within a few days, because i workout daily and get very sweaty. how often should i wash my hair?