Recommended amount of lessons
How many lessons a day do you recommend? I don't want to learn the words poorly by overdoing it, but I also don't want to slow myself down for no reason.
The CEO of duolingo has said a lot people complete the skill tree in 4 to 6 months so that works out to about 2 lessons a day or less on average. I do 1 lesson a day or less myself. 1-2 lessons a day.
Personally, I would propose between 2 and 4 lessons a day, following your motivation and your schedule on your everyday life.
It depends where in the course you are, and how much information you're retaining.
Lessons at the beginning of the courses are generally easier, and I did many more lessons when I was starting out than I do now (now I do 1 every 2 or so days :(). You can try using an independent measure to gauge retention, either flash cards, verb conjugators etc.
Nobody can really tell you what is the best rate for you. If you're studying 6 hours a day elsewhere and you can pull off 13 lessons, go for it!
1-2 practice sessions for revision of old topics, 1-4 new lessons each done a couple of times or each followed by a practice session, a few practice sessions to reinforce what you've learned. About 100 skill points or more in total.
The number of new lessons depends on your level. At first it may be easy to do lesson after lesson, especially if you are not a complete beginner in the language. As you move down the tree, you need more revision to retain all the new words. Besides, the lessons become more and more difficult; you may need to try a few times before you manage to pass a lesson. Therefore there's no shame if you only add one new lesson in a day and earn the rest of your points by practicing old ones. You may also want to have a revision-only day once in a while.
Anyway, these are only my recommendations. There can't be a universal method that fits everyone. Don't worry, you'll eventually figure out the best system for yourself.