Use rolling wave planning and plan your near term activities in detail while approximating the activities that are further out.
The project manager is not a fortune teller.
Immediate next steps are easy to see and schedule. The further out you look, the blurrier the view. It is almost impossible to plan and schedule in detail activities that are to take place months in the future.
The first plans are presented at the beginning of the project. The sponsor may even request a plan to establish the project’s budget before the project is officially initiated. The common solution to the challenge of preparing a plan before all the facts are known, is to prepare the plan using high-level estimates of schedule, budget, and scope.
Rolling wave planning acknowledges that the future is hard to predict and uses higher level estimates of schedule, budget, and scope for the activities later in the plan.
Elaborate in waves.
Rolling wave planning is a type of progressive elaboration. There is more and newer information available as the project progresses, and as it progresses, you start new waves of planning, breaking your higher level plan down into more details.
Make the details match your vision.
Your planning should go beyond detailed and high-level. There are activities in between these two extremes and you should break them down to the level of detail that is visible. Your plan should have a progressive level detail matching the progressive visibility of upcoming activities.
Rolling wave planning (Project-Management-Knowledge.com)
Rolling Wave Planning and Progressive Elaboration (PMcrunch.com)