Departmental duties are as a general rule capped at 20% of study time on average, meaning that you can do them for at most a year of full time work, and complete your studies in five years instead of four. (If you are pursuing a licentiate, departmental duties of 20% result in two and a half years of study). You should keep track of your departmental duties each year.
You should include your departmental duties in your ISP, and revise your ISP every year with this information. If you are teaching a course, or conducting a lab session, for example, there is often a number of working hours that are already calculated that you should spend on the task. You should ask, whenever you are given a departmental task, how much time you are supposed to spend on it, and you should also keep track of the hours you spend in practice.