Here is the relevant German law and a KPMG report describing the rules briefly (from Rob's answer to another related question). The EU also offers (necessarily vague) information on this topic and a list of links to tax treaties between the various member states. There is more than a simple number of days threshold to all this but it seems you would not be a German tax resident under current rules.

Importantly, there is a tax treaty between both countries (there was already another one before that one) and, as far as I can tell, you cannot be considered a resident of both countries at the same time. So, even if you would fall under the German definition of tax residency, your Dutch residency should take precedence (based on your description, it seems to be the country you spent the most time and have the “center of your interests”), which only leaves German-sourced income to worry about.

What counts as German income is defined in this article but I won't venture any guess as to how it applies to your situation or if the treaty makes any difference in this respect. The Dutch Belastingdienst also offers some explanations based on the relevant tax treaties. Note that the rules also depend on your exact status in Germany (posted worker or not). If you want a definite answer, you would probably seek professional advice or at least the opinion of someone more knowledgeable than myself about these questions.

Here is the relevant German law and a KPMG report describing the rules briefly (from Rob's answer to another related question). The EU also offers (necessarily vague) information on this topic and a list of links to tax treaties between the various member states. There is more than a simple number of days threshold to all this but it seems you would not be a German tax resident under current rules.

Importantly, there is a tax treaty between both countries (there was already another one before that one) and, as far as I can tell, you cannot be considered a resident of both countries at the same time. So, even if you would fall under the German definition of tax residency, your Dutch residency should take precedence (based on your description, it seems to be the country you spent the most time and have the “center of your interests”), which only leaves German-sourced income to worry about.

What counts as German income is defined in this article but I won't venture any guess as to how it applies to your situation or if the treaty makes any difference in this respect. The Dutch Belastingdienst also offers some explanations based on the relevant tax treaties. Note that the rules also depend on your exact status in Germany (posted worker or not). If you want a definite answer, you would probably seek professional advice or at least the opinion of someone more knowledgeable than myself about these questions.