I can tell you how it worked for mine - and it wasn't just watch - the toca-boca games for example are fantastic.
It started when they were very little, they'd just watch whatever I was doing on the ipad, or play a game with fishes or fireworks that exploded when they touched the screen.
Once a bit older, the toca games - cutting hair, running a world or a band, swimming around as a fish, or watching youtube according do their interests (eldest went through a phase of watching trainspotter videos - hours of just trains going by!) - they just wander around with it, as they're playing/drawing/whatever it's with them so they can put something on.
When they hit about 4 or 5 they started watching actual TV shows/netflix, interspersed with whatever youtube channel they were into at the time (unboxing videos or whatever).
7-8+ and it started being game playthroughs/tactics/challenges and some more interesting youtube channels like food theory
My only rules were that if I asked them to stop watching something (eg. I didn't like the youtubers who yelled all the time) then they had to, it had to go away when I asked them to put it away (eg. mealtimes), homework had to be done and they had to be ready for school on time (we did have a short phase of no ipad until packed for school, but don't need it now). No ipad overnight (although once in the habit, it just goes on to charge at night and they grab it when they get up in the morning.