Well....that's really easy to determine. Like, really easy. Just describing it is gonna sound funny.
It's not how many pages it is really, but the file size. A story on a computer, is a text/word file and very small. If you hover your mouse pointer over the file, a box will say how big it is.
If it says, say, 300 kb or less, that'll take no time at all. However, if it's in megabytes (one mb=1000 kb), anywhere from 1-20 mb will take from 5 to 20 minutes. I can't imagine you've written a 20 megabyte story though.