I've found that the longer you boil it the better. I have a dozen or more pieces I've found myself, boiled and put in my tanks (usually, screwed or otherwise affixed to a rock or something). I boil for about 30 minutes typically. Some pieces never want to sink (ever). You can drill them full of holes (which I don't like doing) or just always keep them weighted down. When in doubt, throw it out. I once boiled a piece that exuded this tarry crud that ruined my wife's pot (she was mad) and stunk up the whole house. Should've just thrown it out first.