The best time for a co-founder to leave a startup is either 6 months after deciding to move on or when the other founders ask them to.
It's important to make sure that the decision to leave is not based on emotions or the need for a vacation.
The timing of the co-founder's departure can vary depending on factors such as critical expertise, a smooth transition, or toxicity within the team.
It is best to be flexible and let the other co-founders decide the best timing for the departure, and to treat everyone involved with respect and professionalism.