MW - I hope this doesn't sound stupid. But how long can a storm maintain such a strong intensity?
I can take a stab at this. Most major storms do not maintain Cat 4 or 5 strength continually for a period of days. Assuming atmospheric conditions don't change, they tend to fluctuate quite a bit due to eyewall replacement cycles and other forces. You could see a 115 mph storm go to 140 mph, then back to 125, then up to 135, etc. That said, Isabel maintained at least 135 mph winds for three straight days.
With regards to Frances, if she stays in a favorable environment -- and it appears she will -- there's no reason she can't maintain Cat 3/4 strength for a good, long while unfortunately.