How long will it take for a milk protein allergy rash to go away?

Your pediatrician should give you some cream to put on the rash. My kid used to allergic to dairy products too. Yes it will go away. Try soy base formula.

Cow's milk allergy is the most common food allergy in young children. Fortunately, most babies outgrow milk allergies by their second or third year. In the meantime, parents of babies with milk allergies can be reassured that - although there is no treatment that can cure milk allergies - symptoms can be controlled through a dairy-free diet. Babies who develop cow's milk protein allergy may have one or several of the following symptoms: * eczema or skin rash * abdominal pain or cramps * vomiting * or diarrhea How is cow's milk allergy treated? * The breast-fed child If you're breastfeeding, and your child has been diagnosed with a cow's milk allergy, you don't need to stop breastfeeding. In fact, your baby's allergic symptoms can be relieved by simply removing dairy products from your own diet, as well as your baby's. Your doctor can advise you about a healthy, dairy-free diet that would be appropriate for you. * The formula-fed child If your child is diagnosed with a milk allergy, and is being fed milk-based formula, your doctor will probably recommend switching to either soy-based or hydrolysate formulas. But simply switching to another type or brand of milk-based formula won't help, as it's necessary to avoid the cow's milk protein which is found in all regular formulas.

Yes my son is also on Alimentum.the rash could take a few weeks to go away.Is your paediatrician sure your baby has an allergy to casein protein?Did your baby also have bloody stools,bad stomach pain,or vomiting?I had my son tested by stool sample and blood work.If your baby does have a really bad allergy you may want to look at how much casein protein is in the Alimentum.Powdered Alimentum is 17% casein and the ready to use is only 2 to 3 %.But be aware that the ready to use can cause your baby to be very gassy.I would also suggest watching soy as well,soy protein can quite often go along with a dairy allergy.Good luck.
You can always try Enfilac Nutramigen,that is also a dairy/soy free hypoallergenic formula,and if it is a real bad allergy there is a formula called Neocate that has absolutely no dairy at all,not even broken down hydrolysed casein,I used it for a while and I had to get it at Children's Hospital.In Vancouver Canada that is what it is called,could be different where you live.