Pulling in a paceline is driven by purpose, so as has been said, it's purpose-dependent. This is true from a recreational paceline, all the way up to professional racing.
I'm not going into all of the many variables but you can certainly ride slow enough that one person could stay on the front forever, or perhaps they are easily the strongest and also happen to be the ride leader, etc.
You're asking on a forum what's the norm, for a paceline *you* ride in. You would know better than us, so I'm guessing it's not a very organized ride. To answer one of your questions, one rider doing a long pull doesn't mean anyone else needs to...unless it was agreed to, etc. In general, the harder the pace, the shorter the pulls, for a group of that size.
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Ron - Tucson, AZ