Global file have below option:
# DeleteUnprotectedMediaFreq=TimesPerDay
# default: 1
# limit: 1 <= DeleteUnprotectedMediaFreq <= 24 # This period is used to run unprotected media deletion operation.
# If set to 1, operation is performed once per day (00:00), # set to 2 two times per day (00:00,12:00), set to 3 three times # per day (00:00, 08:00, 16:00), set to 4 four times per day # (00:00, 06:00, 12:00, 18:00). If maximum (24) is specified, # operation will be started every hour.
You can set the vaule according to your needs especially when you are using storeonce for backup.