LOL - mom of 5 close-in-age kids, and I'm the opposite! Don't even think about telling me you are bored - the reason I have playdates for my children is so I don't have to entertain (and to be honest, I've been very lax lately with my little ones, since they play together really, really well). I remember one of dd12's first playdate in kindergarten - she went over to this child's home, an only, and did a craft! When the girl came to our house, within the first 5 minutes, she asked me what they should do - um, play?

Since I had them so close together, my kids have just played with each other, since I was always busy with a baby.
I'm with you. Our rule is no tv or computer when friends are over. I prefer that they interact and play together.
DD4 had her 3 yo friend over for 4 hours today (they play together often - almost every week over the summer). And the only times that I was involved - feeding them lunch, cleaning up the mess when they pulled out every single wipe out of a full package (they were playing "cleaning lady", and we don't even have one!), and checking on them when they went to the bathroom.
Four hours is a long time for 3/4 year olds, and it is usually closer to 2 or 3 before they seem to get tired.
If your child and his/her playdate aren't getting along, I'd give a warning and then take/send the playdate home. Sometimes kids are in bad moods, tired, etc. Sometimes it just isn't a good match. I have an 8 yo, too, and, when she was in kindergarten, I took her playdate home early. They weren't getting along (it seemed to be on my DD's side). I gave a warning and then took the playdate home (I felt bad because the mother was sick, and it helped that her DD had a playdate).
I've also canceled a playdate before it started when my DD was acting awful (and she was 8 at the time and she took a 2 hour nap instead of having the playdate).
If a kid extremely rude, I don't usually have the kid back (this has happened maybe once or twice). I explain to my 8 yo that I don't have the patience for that sort of behavior. She seems to understand. I also give my 8 yo a nice lecture of how not to behave at someone's house! At her 7th birthday party, there was a girl who was awful, climbing on a table, smearing cupcake into the carpet - she was acting worse than my younger DD who was 2 at the time. I told DD8 that that girl wasn't coming back.
I agree with the other posters - I'd say 1 to 2 hours for a first playdate just to see how it goes, especially if it is after school (they are tired from school anyway). I don't usually go with 3 hours unless it includes lunch.