You and your manager agree on how many hours you will work. The standard working hours for a full-time employment are 38 hours per week.
More or fewer working hours
Do you have health problems and is it therefore for you only possible to work at certain hours? Discuss it with your manager. You always reach an agreement together on when you start and finish work.
Break during working hours
If you work for at least 5.5 hours per day, you have a compulsory half-hour lunch break.