At what point can a secondary relationship expect not be have to endure couple privilege ?
Well, I wouldn't tolerate it from the start.
Personally, if I find myself "enduring" something shitty in a relationship and can't wait for the day I no longer have to keep on enduring it - then, that relationship isn't working. Sounds soul-sucking to me.
That doesn't mean I expect
all things to be "equal" if I'm involved with a partnered person. What the hell would equal look like, anyway? "You hugged your wife 46 times this week, so now you owe me 46 hugs?" It's not about symmetry.
There is always a "getting to know you" phase of a newly blossoming relationship, and I am not saying that after a few dates I would expect to be on equal standing as a wife of 20 years. However, my being a new lover should not mean that I am automatically less important or less worthy of one's time, respect, and energy than an already-established partner. In my relationships, I want to feel respected, valued, and know that my concerns matter. I want a voice and need to be heard. If I'm seeing a guy, he can consider me a secondary if he needs to use that term, but don't ever let me feel treated like a secondary!
An existing relationship doesn't have to be "downgraded" for a new relationship to be treated equally in consideration and respect. And that equality doesn't mean that everyone will get everything (time, attention, sex, and so on) in exactly the same amounts. It just means that the needs of any partner or lover are prioritized like triage in an emergency room, with more flexible decision-making and an openness/willingness to flow with life's changes and uncertainties, and not automatically based on which partner has the most longevity.
If someone I'm seeing respectfully tells me they can't see me because of an existing work or family commitment, fine. I may be disappointed if I have my heart set on it, but in the grand scheme of things, it's not a big deal. We all have calendars and commitments to abide by, so it's not really a problem to have to work out scheduling issues and have limited time with someone. I also have no problem with designating certain times with one person as private and not to be interrupted by another except for emergencies. However, if I ask for someone's time I do not want to be set aside tentatively "in case" another (more important) partner might need to see him, nor will I tolerate being asked to reschedule to accommodate another partner's preferences more often than is reasonable.
An egalitarian approach doesn't mean that everything is always equally balanced, but it does mean that I strive to never let anyone I'm involved with ever feel that they've been treated as less important to me than anyone else. I would not date people who did not also strive for that in their relationships. So, that means that if someone I'm seeing tells me I'm "not allowed" to do some activity with him, express certain feelings, or make decisions about my relationship with him based on his wife/partner's insecurities or need to feel more important to him than I am, or because she wants only certain things he does to be reserved only for her, that would never work for me.
I run my own relationships and will not submit to a predetermined pecking order established by someone I'm not in relationship with.
For me, each relationship I am in is managed on its own merits, each person's autonomy and agency are equally respected and honored. All my lovers are special to me in their own unique ways, but not in comparison to whoever or whatever else is in my life.
The couple-centric model, with two people clinging desperately onto this idea that their relationship is more "sacred," more privileged, and more important than any others they have, is distasteful to me. I never understand how anyone can allow themselves to get involved with someone who treats them like an appendage to their Holy Dyad, an add-on, "extra," or a satellite orbiting around them. Couple privilege goes hand in hand with OPPs, and leads to all sorts of disrespectful treatment, like one dyad setting rules about how another dyad can conduct their relationship, reading a metamour's emails, or monitoring a partner's communications and activities with their other lovers. It's all about making sure that the couple at center holds the #1 place in each other's priorities and hearts. However, if that is what a couple wants, better to be Open and have casual flings with people who aren't interested in letting their hearts get involved, than to be Poly.
Thank goodness there are couples out there who are not so couple-centric to begin with.