What exactly is a "1 day streak" on Duolingo?
For everyday that you go on Duolingo and practice or do lessons ( you must earn your target XP each day) you will increase your streak. For every 10 days you meet your streak, you earn 1XP x 1/10 of your streak, so after 10 days you get 1 XP, 20 days 2XP, 30 days 3XP ... 500 days 50XP. See http://duolingo.wikia.com/wiki/Streak for more info.
Yeah, except for every 10 days you get a lingot, 20 days 2 lingots, 30 days 3 lingots. Was it a mistake that you said xp?