How long will fever blister rash take to go away?

Blister or Rash? What do you mean by rash? If ur talking about cold sores on or around your lip it takes times. Cold sores are most often from herpes virus type 1. If ur able to start an anti-viral medication like Acyclovir or valcyclovir within the first 48 hrs (the sooner u take it the more effective) it can significantly decrease how bad gets.