How long in advance should I extend my visa in Germany?

There should be no problem. 10 days in advance is enough.

If the documents are incomplete and the visa cannot be extended, you can "make a white note", which means that you will be extended your stay for three months and pay about 30 euros, which is the same as the fee for extending the visa.

If you prepare all the materials, it can be done on the same day. However, it is still recommended that you apply for a visa extension a few days in advance.