Extreme Programming Explained (Kent Beck, 1999):
"XP is designed to work with projects that can be built by teams of two
to ten programmers, that aren't sharply constrained by the existing
computing environment, and where a reasonable job of executing tests
can be done in a fraction of a day"
"Schedule slips—XP calls for short release cycles, a few months at
most, so the scope of any slip is limited. Within a release, XP
uses one- to four-week iterations of customer-requested
features for fine-grained feedback about progress. Within an
iteration, XP plans with one- to three-day tasks, so the team can
solve problems even during an iteration. Finally, XP calls for
implementing the highest priority features first, so any features
that slip past the release will be of lower value."