How long does the primer last?
The standard shop paint is intended to protect the steel for only a short period of exposure in ordinary atmospheric condition and shall be considered an impermanent and provisional coating.
Is the shop-applied paint on a joist a universal primer?
Typically, no. But this is a question for the painter who will be applying additional paint to the joists. The joist manufacturer can supply product information to the painter for his use in determining the applicability of additional paint types and finishes.