My LG recently developed a post viral rash following a few days of high temp and it lasted a day or two before disappearing. She had the rash on her torso- back and front. It seemed to have been worse in the first few hours after it appeared and then got better. Every baby will be affected differently though.
For my son his came up all over I think and lasted about a week. However he also had an eczema flare up caused by the virus which covered up the viral rash initially, so it may have started in one place and then spread.