I have had a blood test to identify food intolerances and am avoiding those foods and eating only safe foods. how long should the hives persist?

Up to 1.5 weeks.: Hives should last about 1-2 weeks regardless of the cause. You would be wise to avoid the foods you cannot tolerate until testing is finished to see how severe your reactions are.

1-2 weeks: Dr. Woods is right about food allergies. If you eliminated all the foods identified by your blood test & 1 or more of them is the cause of your hives the rash should resolve in 2 weeks. If they don't, what then? If hives have been present >6 weeks the chance they're due to a food is 1%. You may also be allergic to a food not tested or to a food with a negative result. Check with your allergist.