Expansion Valve controls the quantity of the refrigerant released by the system in the evaporator. It also regulates the cooled air’s flow in the AC system.
Signs of a bad expansion valve can be improper cooling in terms of too much or too little cooling, hissing sounds, etc. The lifespan of expansion is not fixed as such but depends on the usage. In other words, the more you use it, the quicker it will get worn out.