If during your enrollment you need to change your working hours, for example from full-time to part-time or from 30 hours per week to 22 hours per week, you must submit an application for this. You can find the application here.
You can choose to apply for the change for a limited period or until the submission deadline, and it is possible to make several changes during the enrollment.
The PhD school has a minimum requirement of at least 18.5 hours per week on the PhD project