SCREEN TIME - how much is too much?

Hi All,
I’m having a little trouble with my 23mo DS. Of late, he seems to be “addicted” to the computer, and can sit for almost an hour at a go, watching LR/LM/YBCR etc. When I stop these educational programs and do my own work, he throws a tantrum until I click on “his” programs - the early learning stuff. This happens in the mornings, after his nap, and also in the evenings and night.
He has plenty of outdoor time too (I make sure he does) and Montessori activities and loves it, but the way he concentrates on the PC when his programs are on worries me a little, he doesn’t hear or listen to instruction then, he’s completely GLUED. He can go on and on, sometimes repeatedly watching the same thing if I don’t change it.
I have read on multiple occasions how too much screen time is bad for little kids… I was just wondering if I should be worried about this.