I got a bit over a 500 streak but stopped after that. I felt that spaced-repetition on the words was more effective than whatever DuoLingo was doing, and DuoLingo's lesson for the language I was learning was not well maintained.

The gamification techniques DuoLingo's used made it much easier to maintain the streak. Albeit, it was really easy to just do DuoLingo "to get it done" rather than focusing on it as part of study.