It is impressive, but you can buy items (with the in-app currency you get from finishing lections) that allow you to skip single days (or even whole weekends, IIRC).
The Weekend Amulet is only offered after completing the lesson on Friday, as far as I can tell - you can't just buy them in the store, and they expire after the weekend is over whether you used it or not.
(I'm currently on an 872 day streak.)
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.