The collection of theoretical material can take from a month to two. You should realize that during this period the library is your best friend. Taking thetopicof your thesis, you should scroll the name to it from all sides, look for synonyms.
From theoretical material it is important to look for not just sources, but authoritative sources. The most valuable source is a monograph. Themoremonographs written on your topic, the better. Ideally, you need to study at least 2-3 monographs. You can also search for monographs on a similar topic to your work, for example, a lawyer can familiarize himself with a monograph by a psychologist, a sociologist, etc. depending on the topic of the thesis.