Ideally, the founder’s shares should vest over a 3-4 year period. This is not just in the interest of the investors, but also protects the entrepreneurs in case one of them decides to leave.
In simple terms, if there is a 3-year vesting period, then every month the promoters get 1/36 part of their equity.
For example, if there are 4-founders, and one of them who has 18% equity decides to leave the startup after 15 months because the venture faces significant challenges, then in a 3-year vesting period clause, the leaving founder will get to keep only 7.5% of his 18% equity, with the rest of the equity now available for the company and the board to offer to another person who may be brought in as a co-founder or at a management level to fill in the gap left by the leaving founder.
In case the equity that has not vested to the leaving promoter is not given to a new person, then in the case of an event like a M&A that equity is distributed to all the remaining shareholders, including the promoters in the proportion of their holding in the company.