Teams will need to play planning poker at two different times. First, there will usually be an effort to estimate a large number of items before the project officially begins or during its first iterations. Estimating an initial set of user stories may take a team two or three meetings of from one to three hours each. Naturally, this will depend on how many items there are to estimate, the size of the team, and the product owner’s ability to clarify the requirements succinctly.
Important - Estimate during Product Backlog grooming as well as in release planning. Also some of this will need to be done for items that are new and identified within an iteration.
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