John, obviously the size and "deepness" matters like what caused the blister? Burn? It would matter how deep it burned. The deeper it is the longer to heal.
In most cases I would plan on the blister part being gone in just over a week but not healed. There will then be a scab which will need to heal but will be smaller and less red each day. Then eventually the scab will probably fall off a week later (let it fall off on its own, picking at it will irritate and make it red and last longer because you are interrupting the healing). Then you will have a red area there will slowly go away over time.
My guess is you are looking at 3 weeks