mhsbandar you will need to quickly think of a plot, beginning and a resolution for your story, probably the biggest parts in the narrative. Beginning will have introductions, rising action and so on, plot is just basically the bad thing that happens, the impediment, then the resolution is where everything is fixed. You must keep practicing because i had written a lot of narratives and persuasive pieces so i got quite good at them, and usually would come up with an idea for the story within a minute or so, since i needed most of the time for writing. I wold say 5 mins max for planning time, then you have to start writing because anything less than 15 mins is just wont suffice.