A time horizon is the period between today and when you expect to use money. It gives context to a financial goal; it does not predict what an investment will do.
Try writing down a date or range, what the money is meant for, and how flexible that timing is. A home purchase, school fees, and retirement may have very different timelines. Revisiting the assumptions as life changes can help keep a plan understandable.
This article is general education, not a recommendation. Investments carry risk, and values can rise or fall.