As for biphasic, I think Winston Churchill used a biphasic schedule for much of his adult life. More recently, Glen Rhodes's article on biphasic sleep
suggests in the update at the end that he's been on a biphasic schedule for 2.5 years; the article is from September 2005.
With polyphasic, the only long-term example I've heard is that supposedly Buckminster Fuller slept polyphasically for two years (specifically, one 30-minute nap every six hours). However, I've only read about this in a couple places; other sources talking about Fuller's sleep don't mention this or make differing claims.
I think we're still waiting to see a polyphasic test of a year or more on the Internet.