The Sprint Planning is usually divided into two steps.
In Sprint Planning One the Product Owner presents and explains the ordered Product Backlog items.
The whole team creates a common understanding of the work to be done in the sprint. Next the Development team estimates the work and defines the Sprint Goal In Sprint Planning Two the team plans the work in detail, it breaks down the work in tasks and plans the work. At the end the team commits itself to the content of sprint.
Time window: 2 hours per week sprint duration.
Hints and Tips
The forecast of the work of the team, has a variance. This means that sometimes more and sometimes less is deliverd as estimated.
Caution: If a delivery has to be exaclty what was forecasted, the implementation team will only give its commitment the top of the variance. Thus the team accepts that the lower part of the forecast may not be delivered. See the chapter "Exploit Variability" in D. Reinertsen: "The Principles of Product Development Flow", Celeritas, 2009.