Standbys are 12 or 13 hours usually but can be shorter to stay inside the roster stability scheme. However none of the standby time is duty time so if they call you a minute before that 12 hour period is over you still can work for another 14 hours (or 16 in case of unexpected delays), even an 18 hour split duty is still legal in germany.
Usual standby times are 5:00lt til 18:00lt or 10:00lt til 22:00lt