Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 .
Simplified: The PhD degree usually takes between three to six years of full time study to complete after a Master 's degree .