If we look at standard comics publishing via Marvel or DC, a single serial runs about 30 pages. Typically these serials turn into chapters when combined together to be made into a trade paperback, which typically runs from 75 to 150 pages (although some are much larger). So how ever many serials end up fitting into that.. then some series (like the DC New 52) will have several volumes of trades, so the total could hypothetically be up to 1000 pages for a full story. I think I would consider a run of this length "long form."
Webcomics seem to be a different beast however. I've seen the following terms used:
5-15 pages: "mini comic"
30+ pages: short story
50-100 pages: medium length story
100+ pages: long form
My comic VSEPR has 34 pages, and was printed as a single serial, I feel like both in the printed and webcomic world, most folks would call it a short story.
My comic Goblins of Razard, has 100+ pages at the moment, 7 serials printed, and 1 trade (a second trade will complete the story). It's a little mushy, because online most folks feel it's a long form webcomic, but in the publishing world, only have 2 72-page trades is rather short, so it's probably more of a short/medium length printed comic. Also, I divide up my chapters in sections of 16-20 pages, because our printer requires pages to come in multiples of 4 to avoid blank pages being inserted during print. So my chapters are totally arbitrary on printing, where some webcomics chapter length is totally up to the discretion of the writer.
Interesting question though! 