Duolingo's spaced repetition is poor to non-existent. The whole point of spaced repetition is to prompted to remember information at the right time, but Duolingo relies on you to decide what to do and when. In the new Duolingo layout there is some pseudo-SRS in that the lessons are ordered such that concepts will be presented a few times with increasing gaps between them, but it's still on you to decide whether and when to do new lessons, and once you've done all the repetitions of the lesson you will not see it again unless you decide to go back and revisit it (in which case there is no help to decide when to go back to it).

Personally, I dislike having every app implement (or not) it's own version of SRS, so I combine language apps with Anki. For each lesson in an app, I make an Anki card which simply tells me to revisit that lesson. I then put those cards in a special deck with customised settings with larger review gaps so that I'm not overwhelmed with time-consuming lessons.