I made a mistake on it because everyone was saying I would see a difference if it was dairy intolerance after 2 weeks. I gave up dairy while BF’ing for 2 weeks, and no difference.
7 months later we put my son on Zantac and non-dairy/soy formula – the Zantac helped the pain immediately, the removal of dairy didn’t make a difference until about ***3*** weeks later.
That said, MSPI for us manifested as diaper rashes and yeast infections. It probably exacerbated the reflux, but the difference we saw was really in suddenly having yeast infections settle down.
So yes I would definitely say you need to wait for a few weeks before being sure.