I appreciate when I have the opportunity to draft someone of compatible speed and I am happy to provide that service to another person at another time, but race etiquette does not really apply.
In brevets and other long distance rides I will sometimes be drafting another person for a fairly long time if the course is flat or downhill. At other times different people will be drafting me. If it was a race and you were on a team then it would make sense to rotate turns at the front on a schedule, to maximize performance. But brevets and most other centuries aren't races and it's all up to individuals what they want to do.
BTW, I have always hated the term "wheel sucker". It makes it sounds like one person was leaching off the other person, as if one person's gain was the other person's loss, which is not what it is at all according to the laws of physics. In a team of even just two people riding closely behind each other, *both* will expend less energy to hold the speed they're going at than they would individually. The more people, the more energy will be saved. Of course the person(s) at the back gain more than the person in the front, but all gain compared to riding under "no drafting" rules such as in triathlon. It's not a zero-sum game but a win-win.