Got external hemorrhoids because of preparation for colonoscopy. They are extremely painful and itchy. How long will it take to heal? Will they by themselves? Can I do something so they can heal soon?

Hemorrhoids. It takes months to years for hemorrhoids to form and therefore I do not think colonoscopy prep caused them, it only irritated them. Any cream with hydrocortisone should help cooling them down, avoid prolonged sitting and constipation.
They will heal. Takes a couple of weeks usually. You can use OTC preparations for hemorrhoids which may or may not help. You could also consult with your doctor or a doc here on Health Tap to get the prescription suppositories that may help more.