Who much time do you spend per day learning Swedish?
how much of this time would be spent on retaining/practicing work already learnt from previous exercises and how much time on new material?
My strategy with my main three languages is to first practice all I've already done that has been weak, and aren't gold anymore, then to do a quick "strengthen skills". Then if I feel that "I know this, I want to move on", I start with new skills. If I don't feel brave enough to continue, I just keep practicing the newest skills again, and again, and again. I try to do this everyday, and maybe take an easier skill on days when I feel very low and/or unmotivated. I learn very quickly by doing this, and it takes about 45 minutes to an hour a day, depending on how hard the skill I'm at is. But besides Duolingo, I also try to watch as much movies in the language I'm trying to learn as possible, and watch stuff on Youtube, listen to music, etc. . If you feel that a skill is hard, don't be afraid to practice is often, even if you're past is already.
Personally I don't time myself but rather strengthen the skills I have lost first, seeing if I can remember them as quick as possible. Then I like to go back on the lessons where I don't completely know a sentence or two. In the past, I have done this up to 6 times so far. I do this until I can recall every word that it gives me within a few seconds, and then move onto the next one.
In this way I can feel satisfied that if I see any of the words/grammar in conversation I can reply within seconds, or at least that is my goal. It has worked so far, but there are times when I have to have a little reminder. Overall, I can do from 10 XP a day to 100+ XP in a day, depending on how much time I have free at work.