Before I got into creating comics, I didn't really focus too heavily on the update amount? I mostly just focused on the story at hand, and how ever many panels there were didn't really occur to me.
Now that I am both a reader and a creator, I will definitely say that it can depend on the genre you're working with.
Are you going for a gag comic? Generally those updates are a bit shorter.
Are you going for a in-depth story? Generally, those updates are a bit longer.
I'm working with horror, so my updates are going to be more than 4 panels (which would work better with, say, a gag comic).
Yet, at the same time, it also depends on the story you're going for. Even though I am working with a horror comic, I want it to be concise -- I don't wanna drag the story on with unnecessary details. Hence, my updates probably won't be more than 10 - 15 panels an update.
At the end of the day -- I feel the length of the update shouldn't ruin the story. If you've got a good story going, then the length really shouldn't matter. It's your story-telling skills, and your ability to bring someone back to read more. Because someone who enjoys your story won't focus too heavily on the quantity versus the quality.
Well...at least, that's how I am as a reader.
You will get people who expect you to do "more", and I say "bunk that!".
You go at the pace you know you can do without burning out.