Setting aside timing issues related to the industry and to the shareholders (which also merit careful consideration), the best time to sell a business is after a period of profit growth but before that growth peaks.
Remember that most buyers like to buy growing businesses, so don't leave it until your growth is all in the past. It is particularly important to make sure that the growth potential is clearly visible; the stronger the evidence of future growth, the better. The value of a business can be significantly increased with thorough preparation, starting twelve to eighteen months before the sale process is launched.
During that time, it's well worth developing the growth profile and thinking carefully about potential buyers: who they are and what would encourage them to buy your company. You may want to refine your strategy accordingly.