A friend tipped me off to this info and we found confirmation on other sites, so I'm sharing what we found "on the internet."
Whirlfloc tablets were made to be a commercial-use product for large batches; the maker didn't know distributors were re-packaging for the homebrew market until recently.
Two 2.5 gram tabs (5 gm total) will treat one hectoliter, or 25 gallons, so one tab does 12.5 gallons, so a full tab is more than needed for five gallons. If you're treating 5-6 gallons then half a tab is plenty. And it's carrageenan with a wax binder to maintain the tablet form, so essentially same as irish moss.
There is also information out there that the longer it boils the more it gets denatured and loses some binding power. I'm going to try five minutes next time I brew.