There are no rules, as NYCindie says. What works is what works.
It really depends on the people involved. I live with my husband and my boyfriend. Most of my time is spent with both of them (and most of the time I don't spend with them, they spend together).
I describe my relationship with Dude as being "developing toward co-primary" but I don't spend any less time with him than with MrS. (In my case primary/secondary are "descriptive" rather than "proscriptive" depictions.) Some people are in situations where they may see a secondary (in person) more often than they see a primary (in an LDR)...
I would say that "too little" would be where someone doubts their status as "primary" and "too much" would be where the other person feels that they are getting short-changed...
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe