How frequently does wp8.1 do an automatic backup?

[Windows Central Question]

I have automatic backup activated on my Lumia 1320 (settings + apps), but I am wondering how often does the backup happen? Weekly? Monthly? There is no place to set the schedule.

 

[Harrie-S]

Very good question. There is no setting and I just checked my 925 and it was 19 hours ago. And my 930 was 7 hours ago.

 

[Olaf Goldbach]

My 1320 said "more than a week ago". So I tried doing a manual backup ("backup now"), and after about half an hour it went to 100%, but said that the backup failed and that I should try again. I then deleted the backup on OneDrive and restarted the manual backup, but it seems to take a very long time - stuck on 18% for the past 2 hours - it looks like it is busy backing up Facebook app as it is greyed out on the home screen. Is it normal for the backup to take this long? And I still don't know how often the automatic backup is meant to run?