Hi there!
Honestly, i don't think there is a set amount, and it will vary by the age of the baby, the severity of the allergy or intolerance, how it's administered (i.e., through formula, through actual cow's milk, through breast milk.).
If it is a full out allergy, it would probably only be a couple of times before severe symptoms presented themselves (hives, swelling eyes, wheezing/respiratory issues, etc.), for an intolerance it can take time to build up and then slowly symptoms will reveal themselves - such as fussiness, night wakings, rashes, cold symptoms, loose/mucousy stools, and even stools with blood in them.
You can check out kidswithfoodallergies.com - they are a great community have some serious btdt experts! There used to be a fee for all the boards, but I think they recently dropped it.
I hope this helps somewhat!