Indeed!
40 mins was the aiming point when I used to instruct. Sometimes you would go over, 50 mins max if the student was a struggler or you'd ace through in 30 mins if they were switched on and didn't need much revision from the previous lesson.
90 mins is over kill, they'll be switched off and daydreaming about the lesson ahead fairly quickly and won't take much in!
The only thing you will be achieving in 90 mins is to prove that you love the sound of your own voice.