Depends on what they are watching. Some of this weird crap with bright colors and creepy animations and annoying songs I wouldn't let my kid watch for 5 minutes. On the other hand, your 2 year old could watch 1 hour of boring nature documentaries every day and still get into Harvard.
Watching a video of an elephant eating some grass isn't a better or worse form of entertainment than watching another tot have a tantrum or picking their nose. Which there is a lot of in daycare.