Well the department here at TUM has hard deadline of graduation in 7 semesters. And both mean and median graduation time is 6 semesters. I agree that students take on part time jobs as working students in big firms that fund them, but they don't exceed 20 hours. My analysis is that there is lot of uncertain components that you have to navigate to get your degree (independent research credits, thesis), where what constitutes as sufficient work is subjective. If it were only coursework I would also look carefully at a student who took longer time to graduate :)