I don't think it goes both ways. As an employee, your maximum notice period is always 3 months - which is also the legal minimum required period for employers. Now depending on your position, your employer will probably even have longer period (i.e. if covered by a union negotioted tarif). Additionally, you can't even be fired at will - the employer always has to give a reason. "Shrinking down" due to business requirements would be a valid reason, but that would mean that the company cannot fire you and hire someone else for the same position as long as you got the formal qualification.