It usually takes two weeks to clear up, and avoid heat. Luke warm baths are recommended as the heat can bring out more spots. The temp would be bub's system fighting it, so it is good unless it is really high. If you can't lower it then you need to see a dr asap.

When I had chicken pox I had a severe case that lasted 4 weeks, and required hospitalisation because it affected my breathing and central nervous system. But it was a really bad case.