Two to three years before you want to be finished. The financial clean-up, the management layer, and the customer diversification that raise the price all take that long to show a track record.
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Most of the value created before a sale comes from changes a buyer can see evidence of, and evidence takes time. Accrual financials become persuasive after two or three consistent years, not one. A plant manager is credible after a year in the role, not a month. A second contact at a major customer is meaningful after a renewal cycle. Owners who begin the conversation two years out consistently land better outcomes than owners who decide in January to be out by December, and the gap is not small. Beginning early also preserves optionality: the market, the sector, and a buyer's appetite move, and an owner who is ready can choose the year rather than accept the year that chooses them.