Ideally, you should start planning your thesis one term before you actually want to write it.
There are two advantages if you start planning early enough:
- Only a limited number of students can be supervised by the same docent. These places are distributed on a first-come, first-served basis. If you are late, you might be unlucky, and all the place are already taken.
- You might already know your topic one term in advance and have more time to collect appropriate literature.