How much time per day?
How much time do you spend per day on Duolingo? How long did it take to finish your tree?
I wish I could see the dates I started and (for two of them) finished my trees.
My current daily task is to (a) bring all decayed skills back to gold and (b) do at least two Italian lessons and one Spanish lesson and (c) for any language where no skill has decayed, do one or two untimed 'strengthen skills' or 'practice weakest words' exercises. (Spanish never seems to decay, unlike Italian and German.) When I finish Italian I will increase the Spanish to at least two lessons a day.
When the decay algorithm is stable, this seems to work out at about 12-15 lessons/exercises per day. It is scattered through the day on public transport journeys etc so time is hard to estimate.
Note that the levels get longer with the level. So, from level 1 to level 2 is 60 points but from level 21 to level 22 is 2000 points. There about 320 lessons in the Spanish tree (I counted them a while ago, the figure given here isn't exact) so you can finish the tree with about 3200 points if you lose all your hearts every level and never practise anything. That would bring you to level 12 (if I'm not mistaken). There are 25 levels in total, which you can reach by practising and translating things in Immersion
I think that the other languages have a more lessons in the other trees since you learn more words, but that I don't know for sure. Let's just assume they are 350 lessons each.
It took me about a month to finish each tree which means on average I did 10 lessons per day. The earlier lessons are simpler so I could go through 10 lessons in less than an hour while the later levels (subjunctive past for example) were harder so I slowed down to 4 lessons per hour. Do you remember everything if you do 10 lessons per hour? No, but that's why you should practise the things you've learned of course O:)
So, my estimated guess is that it took about 50-60 hours to complete each tree.
Currently I try to spend half an hour a day practising on DuoLingo :)