Youth Hostels in large cities operate day and night. Other Hostels have individual opening hours - usually they close at 11 pm. Adult guests can often receive a key by giving a deposit.
Individual times of arrival can sometimes be arranged with the Hostel management. Otherwise, reservations are valid until 6 pm. After that, a reserved room may be given to another guest. If you have not made a reservation you can inquire available beds by phone. Receptions open at 5 pm at the latest.
Not all Youth Hostels are open all the year round. Often, there is a closure for necessary renovations between Christmas and New Year, sometimes even in January or February.