Ive used up to 4oz per gallon of just STTP with great results on some trashed carpet.
But as Mr. Cobb pointed out there are other ingredients that need to be there, not only to "DeGrease" but to break down the surface tension and remove other soils that are not oil based.
STTP is what I use as a BOOSTER any more, not my main cleaning agent.
If you want something that will friggen strip the nastiest crap out of any carpet get you a pale of Grease Eraser, But order from Chemisters directly... will save you allot of money.