Does diflucan work for seborrheic dermatitis on the skin and scalp and if so, what's the general dose and amount of time used to treat it?

Diflucan is not used for seborrhea. The usual treatment is a topical anti-fungal, like ketoconazole or ciclopirox, sometimes with a topical steroid used sparingly for inflammation, or a steroid alternative like tacrolimus. Selenium containing shampoos can also help, use a few times a week and apply antifungals right after. Occasionally itraconazole is used, but I have never found it necessary.