The gasket should last years, and the only 2 possibilities for it leaking so soon are 1) it was damaged or not installed correctly; or 2) the person installing the gasket didn't get rid of the oil from the plug wells at that time. Plugs don't need to come out to replace the gasket, and someone could do an incomplete job by not cleaning out the wells. AFA the gasket installation goes, it should be snugged down in at least 2 stages (3 is better), in order to avoid mis-shaping it in one or more places. If someone applies the full torque all at once to each bolt, especially if not using a criss-cross pattern, the gasket is likely to not seat properly, and can start leaking fairly soon (or right away).