According to the SPO, the maximum timespan allowed is 4 months. However, if you write your thesis full-time, the 12 ECTS credits you receive for your Bachelor's thesis correspond to a period of 3 months. The 4-month rule is therefore only intended for exceptional cases (e.g. if you write an exam during your Bachelor's thesis, you do not need to apply for an extension but simply add on the time). When talking to your supervisor, you should make sure that the thesis is really designed for 3 months of working time and that you only write for longer if you still have to schedule study time for exams, measuring devices fail or similar. 
An extension of up to one month beyond 4 months is generally possible upon application to the Bachelor's Examination Board, but will only be approved in very well-founded exceptional cases. 

Kategorie: Bachelor - Studium CIW/BIW