Generally petrol fuel injectors will perform their best when the fuel system has been carefully maintained throughout the service history of the vehicle.  Fuel injectors are a serviceable and are subject to carbon build up and tiny bits of debris that have made it past your fuel filter.  In most cases, you will start to notice signs like poor performance, idle issues, cold start problems, stuttering/hesitation and poor fuel economy.  Injectors also have o-rings and manifold seals that eventually will loose their ability to stop fuel leaking or preventing un-metered air entering the engine.