There is no statutory probationary period in employment law. According to Section 622 (3) BGB, a probationary period can be agreed for a maximum of 6 months. The employer may not set a longer probationary period, but may set a shorter one at any time.
However, there is one exception with regard to apprenticeships. According to § 20 BBiG, a probationary period is mandatory. This must last at least one month and may not exceed four months.