How early can a residence permit/Z visa be extended?

My residence permit/Z type will expire on the 15th of December 2016. I will be working at the same university. Can the residence permit/Z type be extended from the end of August (assuming I go back to my home country in the summer holidays for the month of August) or do I need to go back to my country to extend the Z type on the 16th of December?

Thanks in advance.