One way to figure out how many lessons you do a day is to figure out how many reviews you’re okay having to go through everyday. For a long time I wasn’t sure what to do with the coloured bar that tells you how many items you have at each level (apprentice, guru, master, enlightened, burned), but paying especial attention to your number of apprentice items can be useful. So, figure out what number of apprentice items results in a review number you’re comfortable with (100 seems to be a popular one), then decide whether to do more lessons or not based on how much under or over that ideal number of apprentice items you are.
Looking at the number of reviews for the “next day” can also help you gauge what you should do.
I also asked a similar question when I was in Level 6, so you could check out this thread for the responses people gave me. Just in case it might help: