We did a semi slow wean, starting by halving the dose for about three weeks at which stage there was no significant reflux and then got rid of the rest. I had also heard that there can be a bit of a rebound which is why I tried it this way. I’m guessing you will know if it is not working within a week or two.
I think that it varies widely. Some parents have posted about babies having problems within a few days. Kaelyn seemed fine for 6 months before she started having problems again.