When should we engage in Planning Poker?
Most teams will hold a Planning Poker session shortly after an initial product backlog is written. This session (which may be spread over multiple days) is used to create initial estimates useful in scoping or sizing the project.
Because product backlog items (usually in the form of user stories) will continue to be added throughout the project, most teams will find it helpful to conduct subsequent agile estimating and planning sessions once per iteration. Usually this is done a few days before the end of the iteration and immediately following a daily standup, since the whole team is together at that time anyway.