yeah, but the PCs shouldn't necessarily go barking mad at the first exposure in a given night. The roll would kick in whenever a critter was encountered (though hopefully not every round) and whenever a 'Maddening' power was used on, by, or around the PC. I expect those things to happen over and over again, and the temptation to burn willpower will always be there.
Honestly, regenerating one per day doesn't seem like enough. With this, Willpower becomes a second Health Bar, that's constantly being attacked. It should take weeks or months of therapy to remove derangements and the like caused by Cthulhu Madness, but recovering enough to go out and fight again shouldn't take that long.