My understanding is that excess selenium that is not absorbed will be converted to dimethyl selenide by bacterium normally present in the lower intestine. Dimethyselenide is a very smelly gas. I'd venture that if one's gas passages begin to riek of high heaven, that a situation of excess selenium has been achieved. Given variance in people's systems, body weight, etc., there is no one number of mass units per time period that applies to everyone, as regards what constitutes "too much". Same goes for alcohol, tobacco, firearms, sexual relations, vitamin C, calculus homework, manual labor, writing on internet forums, holding one's breath while swimming underwater, and probably.... everything.