Ok, in your opinion, how slow is too slow and how fast is too fast? I tend to do one new lesson per day, and for the rest of the day I practice last two or three lessons I learned in the past few days. However, I have this feeling I'm going way too slow and that I should maybe do two new lessons per day. I would really like to hear other's experiences on this, thanks :)
I think pace depends largely on what section of the tree you're on and how familiar you are with the last few sections you've learned. If you're acing all your reviews, you could try adding a lesson or two. Some sections introduce new and confusing grammar points that you may want to spend a few days reviewing. The great thing about Duolingo is that you can go at whatever pace is most comfortable to you. So if you feel like you're slogging through vocabulary that you've already mastered, it might benefit you to do more new lessons a day.
Everybody learns at a different rate. Try doing two lessons per day. If you seem to be retaining the material, then all is well, otherwise back off.
With the new lessons you , you'll find you incorporate all the pre-existing completed lessons on top anyway :)