If taking apo - roxithromycin 150mg for 3 days, developing an itchy skin and stopping on doctors advice, how long before the bad reaction stops?

Varies: The duration of an allergic skin reaction varies from days to weeks. Antihistamines like benadryl (diphenhydramine) help and,in severe cases, corticosteroids are used but must be slowly tapered to avoid a rebound flare up. If there is no accompanying rash, the itching could be due to other factors like dry skin. Your physician is best qualified to advise you,however.