Within a month, I'd say. Even my helper noticed it, as she noted how there is no more chunks of hair in the shower after I wash my hair!
I got the tip from my mother, who's been struggling with hair loss since her 30s (so I'd say I have a hereditary issue). She started taking fish oil last year per her doctor's recommendation for her cholesterol, and she noticed it has a pleasant side effect of stopping her hair loss.