How long does it take for a new formula to work?

My baby is 4 weeks old and has been on Enfamil Lipil since birth. But over the past week every time i feed her she cries and fusses after she is done eating. I called her doc yesterday and they told me to switch to Enfamil Gentlease and just to switch right over and not to mix the two. But last night was horrible she cried for over an hour then finally let some gas and a BM out and was fine for a couple of hours. I am not sure if she has an allergy or just a sensitive stomach? Because she doesn't spit up and has a couple BM a day that are a regular consistency. How long will it take for Gentlease to help? And has anyone tried and it helped? Thanks!