ExtremeProgramming "iterations" should be short, preferably three weeks long. In that interval 30 user stories can be implemented. 

'''Accomplishment by Iterations: ''' Each iteration provides a complete cycle, from requirements capture through coding and testing to delivery to the customer - and typically takes just three weeks. Small, incremental investments made in conjunction with three-week milestones drastically cut the risk of a project running out of control.
 http://www.bcs.org.uk/publicat/ebull/may02/briefing.htm 

'''Useful in Planning: '''Time spent planning is time well spent. All projects must have a plan with sufficient detail so that everyone involved knows where the project is going. A good plan provides the following benefits: 
 Clearly documented project milestones and deliverables 
 A valid and realistic time-scale 
 Allows accurate cost estimates to be produced 
 Details resource requirements 
 Acts as an early warning system, providing visibility of task slippage 
 Keeps the project team focused and aware of project progress

 http://216.239.39.100/search?q=cache:fpNzmYY6DxMC:www.projectsmart.fsnet.co.uk/docs/eight_key_factors.doc+frequent+milestones&hl=en&ie=UTF-8

'''Used in a schedule, ''' which should include incremental builds, deliverables, and milestones for monitoring progress and reviewing intermediate products. (Functionality provided in each build should be documented in accompanying text.) If only a high-level schedule is provided in the plan, pointers should be provided to working-level schedules that contain frequent milestones (e.g., two-week increments). 
 http://www.qualityassistant.com/softwaredevelopment.shtml

'''Different Vocabularies for Explaining: '''The first team already developed in three-month increments, with one user-demo in the middle. They viewed the V-W technique as little more than an interesting vocabulary for describing what they were already doing. They were, however, relieved that their way of working could be explained to the project executives. 
 http://www.methodjournal.com/artman/publish/article_18.shtml

'''Visualization Helpful:''' A useful means of project planning is to develop a Gannt chart which shows tasks, timescale and dependencies. (Tasks can often be dependent on each other, requiring completion of one task before another can progress). It is essential to build in frequent milestones (with identified, measurable achievements) in order to be able to effectively monitor the progress of the project. Progress of the project needs to be reviewed regularly against the Gannt chart in order for it to be a useful tool. 
 http://distlearn.man.ac.uk/dldev/developing/projman/project.htm
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Category ExtremeProgramming