Recommendation: 12 weeks. Less than 12 weeks might not allow you to complete the work; anything longer than 12 weeks wades into too much unknown. If you work in two-week iterations, 12 weeks gives covers 6 iterations. You don't want to lose the iterative learning of Agile. Chartering every 12 weeks is a way to come up for air and gain perspective from the iterative cycle and look out to the longer vision of the year goal or semester goal and if you're on-track.
Why this work is important and how it relates to the bigger product.
What is your shared team "elevator pitch" of your work? If you were in an elevator for a few floors with someone for a few floors and they asked what you were working on. And maybe today this is a passing on Teams or Slack instead of an elevator.
What are the goals and outcomes for this period of time?
How will we know we are successful at the end of [time period of charter]?
What are the assumptions and dependencies to consider in this work?
What could potentially get in our way and how can we mitigate those obstacles if they come up?
This could be the core team, or maybe it's a v-team with members from several teams. The team here refers to the main people doing the work for the chartered scope.
What other teams will this team collaborate with or depend on for this work?
Who are the stakeholders of this work?