DD went straight to Neocate at 3 months once her CMPI was diagnosed. Her reflux was horrible, she'd scream for hours every day and barely sleep. She hardly put on weight and was a Failure to Thrive baby. A paed at the children's hospital saw us in emergency and gave us a script for it. Once she was on Neocate full time and I'd completely stopped breastfeeding she was a different baby. She was on a combination of losec & Zantac for the reflux with mylanta given during particularly bad episodes. We were able to stop the meds completely once she settled onto Neocate. She grew out of the CMPI at around 1yo and now at 3.5yo loves her dairy!
DS was also diagnosed with CMPI at 3 months but his symptoms were not as severe and thankfully he never had reflux. I introduced AllerPro comp feeds at 15wks to try to boost his weight gain and then fully weaned at 5 months onto AllerPro as he was still having loads of mucous in his poos despite me being dairy-free for about 8 weeks. We suspect he is also soy intolerant, as was DD.
In my experience the improvement is almost instant. If you haven't seen a significant improvement by 2 weeks I would be pushing to change formulas.