You are principally free to set your official applicaton date, but this should be coordinated with your supervisor, who should give you feedback on whether you are ready, e.g. have a workable project size or need to factor in time e.g. for acquiring relevant source material from abroad (N.B.: Prüfungsordnung: "Die Betreuerin oder der Betreuer hat sicherzustellen, dass
gegebenenfalls die für die Durchführung der Bachelorarbeit erforderliche
apparative Ausstattung zur Verfügung steht."). Of course, you are not officially permitted to work on your thesis until you have officially applied, but that doesn't mean you aren't already conceptualizing your future project.