My son will happily watch back to back lessons in the car. He will happily watch 2-3 lessons at home. If it is a lesson from the share forum that I have selected according to his interest he will watch it endlessly!
He watched ALL of the dinosaur ones 3 days running! I decided to add the sound if he wants those again! Hard work pronouncing them! He is age 4.
At 17 months I would let baby watch 2 lessons at a time, but would allow many viewings throughout the day. Just with eye vision breaks in between. Your baby is obviously enjoying the lessons so make the most of it! children are fickle and in just a few months you baby might decide the only thing he likes is worms or ( heaven help you!) dinosaurs! If you are worried about the screen time print of some word cards and try that too.