I agree with Nikki. You can easily tell if your baby is already tired or bored from learning his lessons
I can tell when my baby has had enough already when he starts looking around, or slides down from my lap or when he rubs his eyes which means he's sleepy.
Just to share it with you, I had already introduced to my son the Alphabets and Parts of the Body flashcards. Now I'm making a recap of the Alphabets to him. This week, we're on the letters A-E. I typed in and printed out the alphabets, one paper for each. Then I also printed out pictures of one-syllabled things (so it's easier for him to understand and say if ever) that start with these letters (e.g. ant, arm, bat, bird, cat, cup, dove, desk, etc), and pasted them on cardboards. Then I teach them to him everyday. For next week, we'll have the letters F-I.
I believe this worked for us because just the other day, my baby said, "EGG"!!!
My mom heard it too and asked, "Did he just say EGG?". I told her I wasn't sure if it was just one of the words he's blabbing right now, but I would like to believe he got it from his lessons!