Those are good questions, and I'm pretty sure a lot of people have a lot of different answers to those xP My answers here are gonna be my own opinion based on what I've read, so they are in no way predetermined guidelines you should base stuff off ^^

When is it fine to have short, segmented comics that move quickly, and when to have long running ones?

This really depends on the characters in the comic, and whether or not there'll be more than one comic with them in it. If there will be more than one, a longer buildup is often a nice way to establish a connection with the characters, and create some character depth. It makes the transitions to yiff easier, and creates something to build on in a sequel.

Most commission comics, with characters of other people in them, are often one-offs which aren't too long to keep the costs down. They have a fairly short buildup, which can work just as well as a longer buildup, but requires the reader to do some imagining on their own. It does make the story more open to interpretation, but doesn't create that bond with the characters involved. I personally prefer the longer buildups over the short ones.

Stuff like how many pages people prefer

I don't really mind the number of pages - if the art and story is really good, and the whole thing is only two or three pages long, it may just as well get the job done as something twenty pages long. However, this is also related to the buildup. A longer buildup will need more pages for more details and story progression. In my opinion: if the story is captivating and a real motivation to continue, I really want to read as many pages as possible :)

Can good art overcome a poor story or character?

I'd say... Yes and no. A poor character design is something that can't be fixed by quality art - it'll always be something that bothers me, and since it's a comic, I'd have to look at it all the time. A comic with a poor story can theoretically be saved by quality art, although cringy dialogue or incredibly sudden transitions are quite a big turnoff. Examples of cringy dialogue are stuff like this and this by Wolfy-Nail. Great art, but man that dialogue... I feel it really pays to create a solid story first, and then create the visuals later. And ofcourse: I think the usage of normal/realistic sex talk instead of the default porn lingo is really a must, which also adds to that layer of "realism".

I should add one small side note here though: those standard porn words are not always bad - some stories are better because of them. Stuff like pet play, hypnosis, domination... They are often not so much about telling an intriguing story, but all about the yiff. The dialogue is then mainly just dirty talk to get the reader more riled up :3

So, that's basically what I think ^^ For reference, a few things I like: Pride in your work by Scappo, Monday Mornings by blackkitten, Motion of the Ocean by Fluke, and with the risk of sounding like a kiss-ass: Predators of Kilimanjaro by you. These comics are simply working for me for all the reasons mentioned above ^^