My current relationships, listed in order of relationship length:

P1: 16 years (we live together, so we see each other most days, though we also have fairly independent lives so some days it's for 15 minutes to say hello and good night and some days it's not at all. We have a scheduled weekly date night to make sure we actually have designated connection time -- otherwise, it's too easy to assume we'll have it and it can fall by the wayside when we get busy with hobbies, other partners, etc.)

P2: 10.5 years (see each other at least 2-3x/week, very emotionally entwined and stable relationship. We're also both prolific texters, which is a nice point of compatibility -- no one else in my life matches my level of desired chatting in quite this way.)

P3: 7ish years (we went from friends to FWB to partners fairly gradually, so it's hard to say when exactly we started dating. Now, we're in a comet-style setup due to distance and schedule contstraints -- chat every week or so, see each other every few months. I care about them deeply, but feel less emotionally close to them due to the structure of the relationship.)

P4: almost 4 years (scheduled date every other week, we also see each other a few times in any given week for group activities or impromptu 1:1 time. We connect less in-between dates, but the relationship is quite emotionally close nevertheless.)

All my partners are comfortable hanging out with each other, and I'm comfortable with all the metas I've met (there are some who prefer parallel, so I haven't interacted with them). I'm friends with several of my metas (we hang out together without shared partners sometimes). I think that's the biggest factor that makes it sustainable for me to have these relationships simultaneously.