How long should we give a new formula to work?

We have switched from isomil (constant fussing and gas) to similac sensitive. She took it ok the had one projectile vomit this evening on the read to feed. I bought the powder that I may try her on tonight since it doesn't seem as rich or thick. How long should I use this formula to see if it helps with the crying and stomach issues? I tried alimentum for one day and she threw up the whole first bottle and would not drink much after that (maybe the taste, she would not drink good start either--made faces). SHe is 1 month old.