The length of your roadmap is a function of how fast you are learning. An established product in a mature market may release new and improved versions one per year, and will have a correspondingly long roadmap. A start-up with new capabilities every week may have a roadmap of no more than a few months. They may even abandon dates altogether and use buckets like, "Now, Next, Later..."
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What works best in your organization? What's too long? What is too short? Tell me a story.