Plan work a quarter at a time. Once a quarter reflect on the team, the project, its progress, and its alignment with larger goals.
During quarterly planning:
Identify bottlenecks, especially those controlled outside the team.
Initiate repairs.
Plan the theme or themes for the quarter.
Pick a quarter's worth of stories to address those themes.
Focus on the big picture, where the project fits within the organization.
A season is another natural, widely shared timescale to use in organizing time for a project. Using a quarter as a planning horizon synchronizes nicely with other business activities that occur quarterly. Quarters are also a comfortable interval for interaction with external suppliers and customers.
The separation of "themes" from "stories" is intended to address the tendency of the team to get focused and excited about the details of what they are doing without reflecting on how this week's stories fit into the bigger picture. Themes also fit well into larger-scale planning such as drawing marketing roadmaps.
Quarters are also a good interval for team reflection, finding gnawing-but-unconscious bottlenecks. You can also propose and evaluate long-running experiments quarterly.