DS (3) watches between 30 minutes and two hours a day. Usually about an hour.
Unlike some of his peers who potter about while the telly is on, DS becomes mesmerised, and I noticed a shift towards arsy behaviour when it was turned off: not just cross about it being switched off, but almost a struggle to readjust to reality.
I'd previously been a telly-after-lunch person. Then, after separating, I'd put it on in the morning while I got ready. Ds would watch up to an hour, not play in that time, and would wake up asking for it, which I didn't like. So I imposed some limits!
No telly until the afternoon. He is now playing while I have a shower, making up all kinds of fab games, chatting through the shower door, etc. Great to see. He understands the rule, and sometimes by about 4pm will ask, "Is it the afternoon now?" We pre-record the shows he likes (all on CBeebies), and have some DVDs, and he chooses what he wants to watch for his telly time. I'm relaxed about him having an hour of it later in the day - as opposed to by 9am - and he seems to appreciate the TV more.
This is what works for us, based on DS's particular relationship with the telly.