How long should I marinate fajitas?

Acid is a timer, not a switch

Citrus and vinegar brighten and tenderize the surface of meat, but left too long they denature the proteins into a mushy, chalky layer. Thin, delicate proteins hit that point fast; thick beef cuts can ride a mild marinade for hours. Shrimpare the extreme case — strong citrus effectively “cooks” them like ceviche, so keep it brief and add most of the lime after the pan.

Always marinate cold

Marinate in the refrigerator, never on the counter, and discard used marinade or boil it before reusing as a sauce. These are quality windows layered on top of basic food safety.