Setting the attributes of the plan
When you create a plan, you give it a name, a start date, and an end date. A sensible name usually includes some description of the time frame covered by the plan and perhaps who or what it's for.
The start date is limited to Sundays only and the end date is limited to Saturdays only. This is simply to create a cleaner interface and provide a more intuitive experience when building and organizing programs within the plan.
PLAN LENGTH
How long should you make your plan? Theoretically, there is no limit to the length of time you can build a plan for. However, the longer your time frame, the more cumbersome your plan will be.
It's usually best to think in terms of phases or cycles that coincide with competition. Many coaches create one plan for in-season, then another plan for off-season. Other coaches create a plan for the whole year. It just depends on the context and complexity of your planning needs.
If we were to establish a general rule-of-thumb, we'd probably say don't plan for much longer than a year. As you are likely aware, schedules can and do change often, so making detailed plans for for excessively long periods of time is usually a wasted effort.
A better approach might be to use the planning tool to lay out a skeleton for the structure and timing of your programs, then make adjustments and fill them in with more detail as time progresses.