No set time frame they usually last for years. You are talking about the fuel primer bulbs that connect to your gas tank correct? If you find dry rot or they start leaking gas or stop pumping gas when you squeeze them then it's time to replace them.
That gray/silver fuel line had me and my father cussing for 2 months on his boat... The inside lining seperated from the outside layer and would colaps on itself cutting off the gas suply when you got over 3/4 throttle.
