What is the usual dosage and how long is it usually prescribed for?

It depends on what is being treated. This is a dose guideline for Diflucan (fluconazole).

For fungal infections:

* For capsule dosage form:
o Adults—150 milligrams (mg) as a single dose to treat vaginal yeast infections.
o Children up to 18 years of age—Dose must be determined by your doctor.

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* For oral suspension and tablet dosage forms:
o Adults and teenagers—200 to 400 mg on the first day, then 100 to 400 mg once a day for weeks or months, depending on the medical problem being treated. A vaginal yeast infection is treated with a single dose of 150 mg.
o Children 6 months of age and older—6 to 12 mg per kilogram (mg/kg) (2.7 to 5.4 mg per pound) of body weight on the first day, then 3 to 12 mg/kg (1.35 to 5.4 mg per pound) of body weight once a day for weeks or months, depending on the medical problem being treated.
o Infants and children up to 6 months of age—Dose must be determined by your doctor.

http://www.drugs.com/cons/diflucan.html