Most keep patches on for 5 days. This helps determine if you have a delayed allergic reaction. For 1 of my allergens it did not show up until day 5.
Do all of you understand the difference between Contact Dermatitis, CD and Allergic Contact Dermatitis, ACD? Most of us have CD and will react within a 30 minutes of less to poison ivy or oak. I will react to cedar or juniper leaves within a few minutes. If one berry or leaf part from the red cedar juniper tree goes down my neck while walking under a tree I know! Within a few minutes my neck will itch and I get a welt. This is CD. ACD is more of a challenge as cause and effect is not obvious because of time delay. For example reactions from chemicals and elements in my toilet paper, dish soap, shampoo, clothing, bedding, flooring, shoes, make up paints, one fragrance, all rubber and so on would not show up immediately. It may take days to develop a visible reaction. Then how can we figure out the source of the cause?
Some even wait 7 days with the patches on. Yes I reacted in some cases in 1 day as I could feel certain sites itch, by day 3 of course itching was worse and by day one site began reacting for the first time and others had a small fiercely itchy blisters at the prick source because of the severity of the allergy. Believe me the discomfort of this test is worth the price because of the knowledge I gained which gave me my productive life back.