I agree that about a conversation's worth is about right. When I'm hostess, it usually works out where I mingle among the groups of conversation, sometimes chatting for just a couple of minutes, sometimes for as long as 10 or 15. Much longer than that, though, and it feels like I'm neglecting my responsibilities as hostess.
As PPs have pointed out, that includes making sure food & drink are replenished, trash & dirty plates picked up, etc. And it also includes more mingling so that no one feels that a certain group is monopolizing my time which can be perceived as rude as LifeOnPluto pointed out.
As the party winds down, it usually dwindles down to my closest friends and we end up sitting around chatting for a long time as they are the only ones left. Knowing that will happen (as it almost always does) I usually concentrate on those I know will not be staying as long during the rest of the party.
As a guest, when I go to a party, I really don't expect to talk to the host/hostess more than a few minutes here and there. It feels to me like the fun of the party is talking to a variety of people. So even as a guest, I tend to mingle about and not stick to the same group the whole time.
Conversations at parties can be pretty unpredictable so having expectations about a specific amount of time that the hosts should spend with each guest is unrealistic. And trying to make it equal is futile. Obviously, I don't time my conversations so that at the end of the allotted time I have to excuse myself mid-conversation to move on to the next group. It's more of a function of the natural ebb and flow of conversations. They pick up, they wind down, and I move on.