Copied and pasted from Homebrew42, on Beeradvocate.com... just for your consideration:
"Whirlfloc was not developed with homerbewers in mind, it was developed specifically for commercial brewers. When the manufacturer of whirlfloc found out that it was being marketed to homebrewers he was shocked. The interesting thing is that when it's repackaged for sale to homebrewers it's always labeled with something like "Use one tablet for 5 gallons of wort. Add 15 minutes before the end of the boil", but this is not at all what the manufacturers recommendations are.
I've seen the manufacturers specification sheet and according to them the correct dose is one tablet for something like 12.5 gallons of wort, and the tablet should not be boiled for more than 5 min. According to them, the longer you boil the tablet, the less effective it becomes. In fact the reason it's called "whirlfloc" is because it's supposed to be added during the whirlpool."