It depends. Some people speed through them, practicing every day, while others take their time only doing a couple lessons every now and then. I feel that I have been going at a moderately fast pace and am on level 13 (almost 14) in German after a little over three months. You could actually probably figure out how long it would take you, if you plan a certain number of skill points to achieve each day. Each level up adds 100 skill points, I believe. Hope that helps! :)

Somepeople (Jibbz) can complete/completed the tree in only a month. For others, it could take up to 200 days or more. It depends how much you review past lessons, and how many lessons you do each day. At one point, I was doing over 10 lessons a day pretty often. I think I completed mine in just over 60 days, but they were broken up (a streak of about 14, then a streak of about 50, which I'm still going on).

I think there is somewhere around 350 lessons in each tree, it just depends on how many you average a day.