You have sold two companies. How do you know when it is the right time to sell?

You should only sell when it is the right thing for the mission and the business. I have a three-axis framework for thinking through this. The three axes are motivation, urgency, and scale. Motivation asks whether the drive to sell is coming from you or from the acquirer. The more it is the bigger company wanting to buy you rather than you needing to sell, the more leverage you have and the better price you will get. Urgency asks where the why-now is coming from. Never try to sell when you are running out of money. And scale describes the size of what you have built, from acqui-hires all the way through to a full business generating hundreds of millions in revenue.