|Usually a month or so but it all depends on your immune system. Could be sooner could be later. Also, depends on how bad it is and how you treat it.How sensitive you are to poison ivy, oak, or sumac and the severity of your reaction depend primarily on your age and on the extent of the first contact or first few contacts with the plant. Other influences include physical activity and immune system function. Some studies show that how allergic you are to the plants may be inherited.