If you add up the total time necessary to complete the tasks from Requirement Gathering to Rollout to Production, you'll find it takes about 9 - 29 weeks to complete each phase of the data warehousing efforts. The 9 weeks may sound too quick, but I have been personally involved in a turnkey data warehousing implementation that took 40 business days, so that is entirely possible. Furthermore, some of the tasks may proceed in parallel, so as a rule of thumb it is reasonable to say that it generally takes 2 - 6 months for each phase of the data warehousing implementation.
Why is this important? The main reason is that in today's business world, the business environment changes quickly, which means that what is important now may not be important 6 months from now. For example, even the traditionally static financial industry is coming up with new products and new ways to generate revenue in a rapid pace. Therefore, a time-consuming data warehousing effort will very likely become obsolete by the time it is in production. It is best to finish a project quickly. The focus on quick delivery time does mean, however, that the scope for each phase of the data warehousing project will necessarily be limited. In this case, the 80-20 rule applies, and our goal is to do the 20% of the work that will satisfy 80% of the user needs. The rest can come later.