age
Entirely depends upon type, age and cycles. for example where i work a 4c on a 20 year old 737 may take up to 20-25 days, wheras on a 5 year old airbus a319/20 will only be 10 days.
Fuselage age & cycles will determine the level of corrosion inspections, NDT checks, lifed component changes etc etc, the list goes on. Even on two identical aircraft the 4c check can be very different.