How long does it take to finish a avrege sized course? If you are diligant?
I've been working on it for about 4 months and I'm probably about a month or two away from finishing my tree. I spend a lot of time reviewing past lessons though to keep them gold. It really just depends on how much time you have available. I generally spend anywhere from 30 minutes to an hour and a half per day on Duolingo during my commute.
It's important to note that it's not worth it to rush through the course. You won't remember as much as you would if you really spend the time studying the lesson notes and making sure you understand the material before you move on. The goal should be retention, not finishing the tree.
It took me a year and a half... but I wasn't the most diligent. I'd say you could probably get it done in a few months if you do several (like 5) lessons a day, and don't review much. I'm not saying that reviewing is a bad thing (you should review,) but it does slow you down. Be aware that the different language trees vary in length.
It took me a little more than three months, but I didn't review enough as I went through. Am considering just redoing the whole thing. It takes time and practice for it to become part of your long term memory.
I'm not finished with mine yet. But I have about 40 more skills to go and I've been at it for almost exactly 3 months. I make sure that my skills are gold every day before I move on to something new. Some days, when there is a lot of review, I might only do one lesson in a new skill. Other days, when there is less review, I might do a full skill. But I don't do more than one skill per day (Except when I got the big update about a month ago and I scrambled to fill everything in).
Anyway, I'm trying to take my time with it and don't have a time window in which to finish. But, I suspect I'll be done in another month and a half. And, I do feel pretty confident that my language skill has improved considerably in this time. It's a good pace for me.