Quite a bit of variation. I've seen markets that take them from just around 1000 words to 7500 words or more. Best to check the guidelines for any particular market you are interested in. Many editors want shorter stories (less than 3000 words or so), even if they'll take longer ones.
it depends on the venue... check the word limits on the kinds of magazines/literary journals you'd like to see your work in... here are the best places to do that: www.duotrope.com www.ralan.com
