Since updating to Sense 6 recently I've noticed that I never see the auto backup icon in the notification bar.
Today I went into the settings (again) to see that everything was enabled and it was. Last sync: 4th of june... (8 days ago) and that might have been a manual activation of the backup.
I did a factory reset after updating to sense 6 so everything was 100% stock while setting it up.
Before the sense 6 update I could see it backup every other day, not sure how often it actually did it but I saw it regluary.
I'm using drive, not dropbox.
I'm usually on wifi