I'd say too much is when she wants it all the time. She needs to be stimulated by other things as well. Its not a bad thing letting your daughter watch tv, and its not bad letting her watch Family Guy because she's too young to understand the actual content. Just be aware of how much time she is spending watching television and encourage baby friendly shows.
My son is 4 and we watch CSI, Simpsons, Family Guy. Good lord, people need to relax. It's not pornography or "Hostel", it's a cartoon.
How much is up to you and I think it's subjective. I don't think there's a particular 'one answer' here. Most of the time my son and I are on the go doing things outside or playing around the house. Other days we relax and really do basically leave the tv on all day. The point is that I think it's "too much" if that becomes all you two do. So long as you're still attending to life as normal and doing what you have to do and being active outside of the house, it doesn't matter if the tv is on 24/7 when you are inside to me at least.