@Lovethetimeyouhave
YANBU.
Six hours at anyone's house is too much!
Some people though, seem to think nothing of spending all DAY at your house (and expect you to spend all DAY at theirs...)
I have 2 BFF. They don't know each other, but I am good friends with them both. I meet both of them (separately,) once every 5 or 6 weeks, and we either meet in Starbucks, or for a pub lunch, OR I go to their house or they come to mine. Never, ever EVER do we exceed more than 2 hours together. It's plenty.
My DH's cousin lives 70-75 miles away, and visits us twice a year, and we visit her (and her DH) twice a year, and OMG I dread those visits from them. She sometimes comes alone, and sometimes comes with her DH, but EVERY SINGLE TIME, she gets to us for around 10am, and stays til bloody 8pm.
TEN HOURS every single time. I have nothing in common with her and don't even like her that much. She is verrrrry right wing/bigoted/racist, and not a terribly nice person. Her DH is OK oddly, but I don't like her.
Also, we have a couple of other relatives who only visit every 3 or 4 months or so, and when they come, they don't seem to know how - or when - to leave. They come around midday and are still there at 7pm. It's such hard work, trying to keep conversation going ALL BLOODY DAY! 
When we visit theirs, we have usually had enough after 3 hours, (I mean these relatives AND DH's cousin,) and they ALWAYS mither and complain when we try and leave, by complaining (like a pp said) that they spent a lot of time preparing, and cooking etc etc, and we're going already! With a kind of a
look!
So we end up forcing ourselves to stay another couple of hours. It takes us 2 days to recover, as it's so stressful spending 5 hours or more with them (on top of the travel there!!)
It's making me HATE visitors, and HATE visiting people too.
We used to know a couple (in our area - left a year ago thank fuck!) who would come around (uninvited!) and stay for 3, 4, or even 5 hours. Got to the point when we started saying 'yeah you can pop in for a bit' (if they came at 1pm,) 'we have to go out at half two, so that gives us an hour and a half!'
So we would be polite and let them in, but knew we could get away after an hour and a half. So annoying that we had to go out when we didn't want to, (and STAY out in case they 'caught' us, coming back too quickly.) 
I don't know why people have to pop round and meet up anyway. Pisses me off so much. Just say hello when you bump into them in public, and meet for a coffee at a mutually convenient place once a month or so. Why do people want to spend bloody HOURS at your house, (being entertained by you!) And then have you at THEIRS for hours on end?
Oh how I fucking hate it.
As I say, it's making me hate visitors (and visiting too,) and is making me very anti social.