This is a question I’ve been thinking about on and off for over two years. I’ve realized that the amount of prosody that’s appropriate has similar answers to questions such as what level of other things like violence, exposition, dialogue, etc. are appropriate: it depends on purpose and context.

On some occasions I’ve told myself that the right amount of prosody needed to be a crossing between the writer’s taste, that of the general population, that which publishers desire, the writers purpose, etc. We’re living in a time when ornamentation is considered elitist although that has been replaced by other factors which make literary writing inaccessible to most, so that idea of compromise would allow for too little leeway.

Then I thought about concluding that there is no objective answer and that the correct amount of prosody depends on the artist’s subjective taste; but that seems like a cop out to me. I feel like if there’s not even a little bit of objectivity in art, then the word, art, becomes meaningless.

Now, I’m considering that the right amount depends on the context and reason. Let’s take this example:

>    Looking at Kate, he was stunned and saddened more than he had ever
> been, gazing at her smoky quartz eyes shining darkness as pained as
> children’s eyes glow joyous and hopeful; her eyes, dark as sadness
> manifest where light is as a darkness visible, a smoldering grief
> unnoticed by those who would see only her eyes’ laughing twinkle and
> miss the breathing embers burning in a gaze infinitely wrathful,
> helmed embers fierce in preparation for battle, fuming embers
> emanating from caverns of anguish deep within her soul, caverns
> descending into fathomless pitch where black memories drift in eternal
> turmoil.

That’s in the first chapter where Kate is trying to find someone to help her with her suicide, someone who “gets” her.
The person looking at her needs to be someone who sees more in her than anyone else. Normally, describing eyes for so long would be excessive; but in the context that Kate is searching for someone to be the person who assists with her death, he needs to be someone who sees very deeply into her.

If I’m writing in third person omniscient, then I can clarify by explaining that she noticed his sadness and surmised his depth of insight and leaned further towards choosing him.

To me, that context and purpose makes something like this prose which would normally be excessive become appropriate.

Let’s look at the other example. The lofty prose and esoteric vocabulary are appropriate because it is describing the characters as lofty divine and celestial beings. The readers put off by that wouldn't be the targeted audience. 
However, I recognize that the targeted audience would be so small that I will pull back somewhat by changing some words: pulchritudinous to precocious, corundum crown to bejeweled crown; and then shortening it by removing redundancies

>    In her school she flew amidst a pitying of turtledoves, precocious
> girls who to some were genial, greeting them with amities and loves,
> au fait but demure, luminous pearls frolicsome and cordial dancing
> around them like noir fay in couture but then flittering away like a
> swans’ bevy joined by belts of silver taking flight together.
> 
>    Such were Kate and her corundum crown of aureate camarilla, Artemis
> and her starry Pleiades in radiant illustration scintillating
> desirably but unattainable in an unreachable empyrean and dancing as a
> constellation of sylphs in a moonlit glade.

   

Since the story uses the template of the Persephone myth and other Greek / Egyptian / Celtic / Sumerian myths, then the swans joined by silver belts referencing Cúchulainn and the explicit Artemis reference position the story in the mythic realm while the prosody creates an ethereal dreamlike atmosphere.

The rhyming words, e.g. turtledoves and loves, genial and cordial, heavenly and momentarily and unexpectedly, precocious and luminous, etc. make me prefer it to that which I’ve read from Tennyson except the parts that I stole from him and other poets such as sylphs in a moonlit glade (actually most of it to some extent).

Will people choke on the antiquated mellifluous style? I’ve noticed that among acquaintances I've requested feedback, those from Western cultures hate it and those from Eastern cultures have enjoyed it; but yes, people who will be the most likely readers won’t like it.

Should one care if readers will appreciate the intensity of prosody? Concern over readers reaction is another factor in determining the appropriate level. Do I care that readers won’t like it? Yes, unfortunately I will.

Can I live with that? I hope so. That’s another factor in determining prosodic presence.

Will potential publishers reject it because of its ostentation? Probably. Publisher reaction might be a concern for you as well.

Will I be willing to compromise if a publisher requests compromise? I don’t know. That’s a question each writer will need to ask himself or herself.

So, what’s the answer to my question: How much prosody is excessive and how to determine that? I hope that what I’ve suggested here, although I have not provided a direct answer, implies direction or at least food for thought, namely that context and purpose play a significant role in deciding.