My only rule about side stories is that they should be interesting. Theoretically that means any amount is fine, but in practice I find that I can only tolerate a filler arc every so often.

Occasional fillers are fine as long as I don't feel like they're 'interrupting' my reading of the main story, ie they get released 'alongside' and not 'instead of' a main story chapter, or they're in the omake (or, equivalently, between main story arcs), or they don't scream 'filler' at you (e.g. 'normal day in the life of the characters' type thing which serves as extra characterization), or something of that sort...
Even with all that, they'd have to be pretty infrequent though.

I only like side stories if it helps develop the plot or characters in some way, but couldn't really fit into the main story-line. Those are usually worth reading and aren't filler-ish.

I usually skip them, but some are nice.

Yeah these....I skip them when im in the heat of reading a realllllly good manga.....sometimes though..i read them....if they're good enough to be read

Personally i like side stories if they develop the characters or are somhow related to the main plot. Aslong as they keep to that any ammount is fine, but i choose every once in a while since i prefer main storyline.

I'm not a big fan, but I don't mind them if they're not too long or frequent. I'd prefer it to stick to the main plot as much as possible, but I understand a little break from the plot is needed once in a while.

As long as they don't happen too often and don't last for too long, it's good to have a distraction from the main plot.

One chapter side story isn't always bad, but too many (especially an entire arc) ruins the story. Too much side story crap tells me the mangaka doesn't know what to do with their story, like their flying by the seat of their pants. An entire arc (say 10+ chapters) usually ruins the story IMO. That's when series get dropped off of my reading list. Don't care how good is was before and don't care enough to find out what happens after. Bleach is the perfect example for me. After Aizen died, they had that crap with the swords gaining physical bodies and the fullbring. Couldn't take the stupidity of it when the fullbring chapters came around and it was like Kubo was just wringing money out of his fan base. Nothing of any importance happened. Nothing really changed. No real character growth. So one of my top 5 were dropped overnight (or more accurately, "overweek"). I check out a few pages every month or two and still feel the same way. Something about quincys. Haven't seen Ichigo or his friends in about 6 or 7 months. I'm glad I dropped it every time I take a peek. To me, Bleach ended with the death of Aizen.

i usually skip any side stories if skipping them wont affect my understanding of the main plot
so yeah i'd rather the mangaka sticks to the main plot

Don't like them.
Sure, I don't mind a chapter or two about a character or something that contributes to the story sprinkled in a long manga, but I really can't stand the volume-long deviations.
It's a pain going through them sometimes.

So... Stick to the main story, dang it. >_>