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" Vee-plus with -
MrS: hetero, probably mono male, my live-in husband (together for 21 years, married for 17)
Dude: hetero, probably poly male, my live-in boyfriend (of 2 years; friends for longer) and MrS's best friend (for several years longer than that)
VV and MsJ: bisexual women with male primaries, LDR FWBs (of 19 and 7 years)
My poly blogs on this site:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe