I think it depends on the topic. A novel needs to be about the same length, whether in print or as an ebook. I have an academic paper produced as an ebook on my Kindle and it's about 8 pages long. I also have many non-fiction books on my kindle too and they are pretty hefty, just as they are in print.
If you are wanting to produce an ebook on a topic, for instance as a free download on a particular subject, then somewhere between 10 pages and 30 pages is probably about right. At 10 pages you can call it an "article". At 30 pages you can call it a "report", above that you could call it a novella (for fiction) a short novel or teenage novel (they are usually shorter). You could also use the terms "hefty tome" for a VERY long book.