I can't find anything that suggests the MCC certificate has a validity period. Its not quite true to say that King Air flying doesn't build hours to your ATPL. Hours are hours, you just can't count them to the 500 hour multi-crew requirement. In reality you would be very unlikely to need an ATPL with your first airline job, nearly everyone starts with a CPL and, by the time you are ready for command, you will have much more than 500 hours multi-crew. The bigger nuisance is to have to do an MCC course even though you might consider you are by that time quite experienced in multi-crew ops.