It's most unusual for me to disagree with Batmom, but this time I do, a bit.
If a gut is upset, it can take a long time to recover. And a sensitivity can take a while to show up. So while the basic reaction (better or worse) is usually clear fairly soon, you might not see the full effects (good or bad) for quite a while.
I do switches fairly slowly, not for adjustment time, but in the hope that I'll see a negative impact before the results are too dramatic.
But then I have one dog with two types of severe IBD (diagnosed via scope), one with probable IBD (not officially diagnosed), and a few others with food sensitivities. One gets...careful.