Why are my mosquito bites lasting so long? Usually they go away within an hour. Now three days later it's visible and still itches.

Allergy. Most people become allergic to mosquito venom after a bite or two. with each bite eliciting a larger reaction than before. It is possible that the lesion and itching may even become progressively worse over the course of a week. Try topical hydrocortisone plus anti-itch cream on the site.