Notice periods in a German job
Two weeks during probation, four weeks after. It grows the longer you stay, and the rule is not symmetric.
A job offer spells out the salary and the start date, but the notice period decides how fast you can actually move when you want to leave. I find people rarely read it until they need it.
The statutory floor
German law sets a minimum, and most contracts just point to it. During the Probezeit, notice is two weeks on either side. Once you are past probation, the baseline is four weeks, timed to either the fifteenth or the end of a calendar month. For you as the employee, that four-week figure usually stays put no matter how long you stay, unless your contract says otherwise.
The part that grows
For the employer, the notice period lengthens with your tenure. After two years it becomes a month. After five years, two months. It keeps climbing for very long service, up toward seven months. This asymmetry is on purpose: the law makes it harder to remove a long-serving employee than for that employee to leave. Your contract can agree longer notice, but it cannot drop below the statutory floor, and any longer notice it sets for you cannot exceed what it sets for the employer.
A practical version: if you are interviewing while employed, work out your real notice before you promise a start date. Four weeks tied to month-end can mean six weeks in practice, and a new employer who knows German contracts will expect that.