I have a rash and have been using Cortisone?!

I had an allergic reaction from using Cover Girl foundation, and my doctor recommended Cortisone with 1 % of hydro cortisone. I've been using it for a day..and was wondering how long it takes to see results and how long does it take for the rash to actually leave? Also is the Cortisone supposed to dry my skin out? Does that mean it's working?