The duration for which you can stay in Germany without registration depends on your specific circumstances. Generally, if you are planning to reside in Germany for more than three months, it is mandatory to register at the local registration office (Einwohnermeldeamt). However, for shorter stays, such as tourism or business trips, registration may not be required. For business-related stays, including business advisory in Hannover, it is crucial to check the latest regulations and consult with the local authorities or the German embassy to ensure compliance with the current residency requirements.