I tried not to with DD1, but by the time she was 2 and I was heavily pregnant with DD2 she was watching a lot. She became a bit obsessive for a while, but these days (age 4) she can take it or leave it- there are specific programmes she will ask for (current favourite is Tom and Jerry) but she won't just watch it aimlessly.
DD2 (just turned 2) was exposed to it much more than DD1 at a young age, but she is not bothered in the slightest and gets bored by it v quickly.
We don't have any set rules, I don't limit it as such but I wouldn't let them sit watching for hours- we're usually out and about anyway.
Funnily enough we went to soft play earlier in the week with a friend who is v strict about TV. While my DD1 ran off to the big slides, her DD was transfixed by the giant TV screens they had and would not go and play, despite endless pleas from my DD1 and her mum!