Depending on the specific field of study , completion of a PhD program usually takes four to eight years of study after the Bachelor 's Degree ; those students who begin a PhD program with a master 's degree may complete their PhD degree a year or two sooner .
The PhD\/DPhil degree usually takes between three to six years of full time study to complete after a Master 's degree .
Young giraffes become mature when they are 4 years old , and they are fully grown when they are 6 years old .