It depends on the person and the story. One person might write a short story in one day, another person might take months to write and rewrite to get the story perfect. There's no standard amount of time when it comes to writing.
This depends on your iOS and settings. If you are on iOS 13, go to Settings > Safari > Downloads and check what is your download location, it should be...
Short stories, are as they say, short. But it greatly depends on the age group, like for preschoolers they have few pages but huge words and simple sentences with colorful...
If you would like to write long stories instead, go for it. You could also just write some scenes (dialogues, battles, descriptions, etc) for a long story. Skills will come...