Playdate Etiquette

A couple of playdates today have got me thinking. What is the etiquette for playdates these days? Baby Girl is four now and has started going to playdates where I can leave and come back later. We've also started hosting playdates and some questions that have bubbled up in my brain are:

1. How long is a 4 year old playdate? Is 2 hours enough or would a whole morning / afternoon work?
2. What's the food rules? I agree that there can always be a snack. But if you're invited for a playdate that starts in the morning, should you assume you're staying for lunch too?
3. In line with food rules, what are the treat rules? Can you or should you offer candy, ice cream, cookies, etc?
4. Can TV watching be included in playdates? Here's where I'm struggling. It seems to me if you're having another kid over to play, that they should be playing, not watching TV. Should I be concerned when my daughter goes on playdates and they watch TV?
5. What are the reciprocation rules? If we're invited one week, should we invite back the next week or soon after? Within how much time before you should definitely reciprocate? This is obviously easier for me as a stay at home parent. It feels like a dating question - how long to wait before you call.
6. I have two kids. So is it fair to ask to have both kids hosted if the other family is a two kid family too?
7. What about discipline? I think it's fair to enforce your own house rules regarding manners and such, but what if things get out of hand?

It's seems kind of complicated. Or maybe I'm just making it more complicated for myself.

Any thoughts?