I don't know how I originally pieced that together. Here are two references, one talking about whole-body elimination half-life, and the other about plasma selenium optimization. I look at selenium as I do iodine -- getting blood levels stable is one thing, and giving the whole body a chance to build up the stores it wants is another.
This is is one of those Google books results. The book is "Selenium in Food and Health" by Conor Reilly. The first sentence of section 2.4.6 reads, "The biological half-life of selenium in the human body has been estimated to be approximately 100 days (Griffith et al., 1976)." I think it takes roughly 3-4 half-lives for steady-state to be achieved. The original citation says the author's last name is Griffiths. The full pdf is available here. I confess I can't figure out how the "100 days" was arrived at, based on what Griffiths writes. This is not my strong suit.
On the other hand, plasma levels of selenium plateau readily, taking about 6 weeks with daily supplement of 50-100 mcg and about 10 weeks with daily supplement of 200 mcg. (Intake from the diet is considered when establishing need for supplementation.) Following this plateau, at a selenium plasma level of 120-150 ng/ml, the authors found no additional benefit because higher levels did not increase selenoprotein P activity, which was used to gauge selenium effectiveness.