Apparently the initial implementation (iOS 8) randomized it every time the screen sleeps or wakes. I wasn't able to find any recent documentation detailing the current behavior - probably similar?
```
iOS uses a randomized Media Access Control (MAC) address when conducting
Wi-Fi scans while it isn’t associated with a Wi-Fi network. These scans could
be performed in order to find and connect a preferred Wi-Fi network or to
assist Location Services for apps that use geofences, such as location-based
reminders or fixing a location in Apple Maps. Note that Wi-Fi scans that happen
while trying to connect to a preferred Wi-Fi network aren’t randomized.
``` iOS uses a randomized Media Access Control (MAC) address when conducting Wi-Fi scans while it isn’t associated with a Wi-Fi network. These scans could be performed in order to find and connect a preferred Wi-Fi network or to assist Location Services for apps that use geofences, such as location-based reminders or fixing a location in Apple Maps. Note that Wi-Fi scans that happen while trying to connect to a preferred Wi-Fi network aren’t randomized.
.... [continues with all the cases] ```