Students form EU member states (exception of Croatia),Iceland, Liechtenstein, Norway and Switzerland can work as much as they want.
You are allowed to work for 120 full days or 240 half days within a calendar year. However, this work may neither impede on the purpose of your stay - studying - nor may it extend the duration of your stay.
Only the actual workdays are counted, not the validity period of your employment contract. If the work does not exceed four or five hours per day, it is counted as half a day. The regular working day of the other employees at the same business serves as the measure here - a regular working day of 8 hours means 4 hours half day; a regular working day of 10 hours means 5 hours half day.
If employment is not spread over a longer time period but occurs continuously (e.g., during semester breaks), only those days are counted as full or half working days during which the student has actually worked. This means that the weekend is not counted in a continuous period of time as long as you did not actually work on the weekend.
Employee and employer must keep track of the number of hours worked within a calendar year without a permit so that the 120 full or 240 half working days are not exceeded. You must properly document the times during which you actually worked.
Working as an undergraduate or graduate assistant (academic student employment) can always be carried out without requiring approval and without time restrictions. Academic employment includes occupation with a department at the university and other academic institutions as well as collaboration in research projects. Academic student employment also includes work for organizations affiliated with the university (e.g., tutoring in the residence halls of the Studentenwerk).
If you have used up your quota of employment that did not require a permit or if your employment does not fall under the categories listed here, you will need a permit for your occupation. You can apply for this permit with the Foreigners’ Registration Office or - if you are a student from Croatia - with the Employment Agency (Agentur für Arbeit).
Employment for students that exceeds 120 full workdays or 240 half workdays may only be granted as part-time employment. The occupation can only be approved if it does not change the purpose of study and if it does not pose a significant obstacle or delay in completing your degree. The Foreigners’ Registrations Office issues work permits on a case-by-case basis. The Federal Employment Agency must usually also approve the occupation (the Foreigners’ Registrations Office will make this request on your behalf).
No permit is necessary for this type of internship. Because it is required for your studies and thus included in your residence permit, you do not need a permit even when the internship is paid. This also includes the completion of a final thesis at a company. The 120 full and 240 half workdays without permit are not affected and may be used in addition.
They are not part of the curriculum and you will need to make sure that the 120 full workdays have not yet been used up by other employment! If the 120 workdays have already been used up, voluntary internships require a permit, even if they are unpaid. In that case, you must apply for the necessary permit with the Foreigners’ Registration Office.