Good question. That depends on what you’re writing–there are no hard and fast rules, but there are guidelines. Articles should be long enough to get the point across and short enough to keep a reader’s interest. Try to keep it to a page or less; two pages maximum. Exceptions exist, but generally, keep it short.
Short stories are–just that, short stories. 10,000 (ten-thousand) words or less is ideal. That works out to be about 35 pages, give or take a few.
Longer fiction and non-fiction books should be less than 120,000 words or so. To put that in perspective, I was working on my fiction book a few years back, and at 146 pages it was 42, 891 words long. So a book with around 120K works would be around 350 pages, approximately. Longer pieces should be split into a series.
Do you write brochures, columns, magazine fillers, etc? The people who hire you will let you know what their preferred lengths are. Stick with their numbers.