Building a Product Vision requires prior "homework": deep qualitative customer research validated broadly with quantitative data (the Trainline approach), plus clarity on corporate strategy. A short "vision sprint" only compresses well if these ingredients are already gathered — it's a synthesis exercise, not a discovery one.
Corollary: don't attempt a vision sprint while corporate strategy decisions are still pending (e.g. due in ~3 months) — sequencing matters, since the vision needs to align with strategy rather than guess at it (see Mission → Vision → Strategy → Roadmap Hierarchy).
In practice a vision sprint runs about a week — a condensed team exercise to articulate the vision once its ingredients are already in hand. Running the sprint before those prerequisites (validated customer problems, corporate-strategy clarity — see Three Core Vision Ingredients) are gathered produces a groundless vision no matter how well the week itself is facilitated.