Federal Leave Act
In short: The Federal Leave Act (BUrlG) governs the statutory minimum leave.
In more detail: It is 24 working days for a 6-day week, so 20 working days for a 5-day week. Collective or employment contracts often provide more.
In Depth
Other rules
The full entitlement arises after 6 months of employment. Remaining leave lapses only if the employer pointed this out in good time. Leave and illness: sick days with a medical certificate are not counted. For trainees under 18 the JArbSchG applies. See BBiG, Working Hours Act.
See also: Working Hours Act, Youth Employment Protection Act, BBiG