On Bluefin it looks like it’s 4:00am +10 min. It will also run shortly after booting, if the system was off at 4:00am.
The config file isn’t sudo writable, so not sure if it can be overridden. It might be possible to create another systemctl timer that runs at, say, 10:00am, or whenever your device is normally on.
systemctl cat rpm-ostreed-automatic.timer
# /usr/lib/systemd/system/rpm-ostreed-automatic.timer
[Unit]
Description=rpm-ostree Automatic Update Trigger
Documentation=man:rpm-ostree(1) man:rpm-ostreed.conf(5)
ConditionPathExists=/run/ostree-booted
After=network-online.target
Wants=network-online.target
[Timer]
OnBootSec=1h
OnUnitInactiveSec=1d
Persistent=true
[Install]
WantedBy=timers.target
# /usr/lib/systemd/system/rpm-ostreed-automatic.timer.d/override.conf
[Timer]
RandomizedDelaySec=10m
OnCalendar=*-*-* 4:00:00
Persistent=true