I don't know if there is a difference between Europe ana USA, in USA the contract usually says that the rate is good for 4-28 days... Or something like that...
I would assume that you would be able to keep it up till 28 days...
I have extended rentals in the past, I called the 800 # to modify my reservation and tell them I am keeping it for longer.
I once kept the car a day extra in Israel, the location called me when I am returning it... I said next day, it was fine.