How long does it take when switching a babys formula for the old one to be completely out of the babys system switched from w/ iron to soy cuz bad gas?

About 5-7 Days: Although each child is different, it is quite common to see some GI changes with switching formulas - especially over the next week. I would make sure to give some time to allow the child to adjust to a new formula before making a decision on switching again - at least 5-7 days. The child's digestive system needs time to adjust to the new properties of a different formula.

3-5 average: The transit time for early infancy varies some, but it should clear within 5 days. Be aware that soy formulas are based on soybean protein and many times these kids actually have more volume and gas.