You receive the equity not for being around on the day the decision to found a company was taken, but for being around from day 1 until the company stops being a startup. (Well, some of the equity may also be for other reasons.)<p>The other founders expect you to be around during that period. One commits in part because the others do.<p>When you leave earlier, you should estimate how much of the period is left and talk to the others about the portion of equity that corresponds to that period — how big it is and how you can use it for the betterment of the company. Are some new people taking over what you would have done? Perhaps they should get some of the equity, on fair terms?