
Developers
estimate how long the stories might take to implement. Each story will
get a 1, 2 or 3 week estimate in "ideal development time". This ideal
development time is how long it would take to implement the story in
code if there were no distractions, no other assignments, and you knew
exactly what to do. Longer than 3 weeks means you need to break the
story down further. Less than 1 week and you are at too detailed a
level, combine some stories. About 80 user stories plus or minus 20 is
a perfect number to create a
release
plan during release planning.

Another
difference between stories and a requirements document is a focus on
user needs. You should try to avoid details of specific technology,
data base layout, and algorithms. You should try to keep stories
focused on user needs and benefits as opposed to specifying GUI
layouts.