The update was supposed to hit unlocked Lumia 1520 devices in Hong Kong last week, but was held up over deployment issues.
Per an email sent to Windows Central Forum member terencechan:
"The Denim OTA delivery to Lumia 1520 Hong Kong Country Variant, OS version 8.10.14234.375 and Firmware version 02540.00019.15053.26006 had effectively started over a week ago, but I just received confirmation from our software updates deployment responsible on the Technical Support Operations team in Finland, that some unexpected issue is blocking the chain."
Initially it was expected that the update could take a few days to clear and get released, but numerous forum members are reporting that they've started receiving the Denim update themselves. To download it for yourself, open Settings, drop down to Phone Update, and then tap on 'check for update' to start the process. Users in the Preview for Developers program will be able to download without any fuss.
Lumia Denim brings the requisite fixes and tweaks to Windows Phone, as well as several new features like Hey Cortana passive listening voice commands and Lumia Camera 5.0 support with HDR, Dynamic Flash and faster shot-to-shot times.
Thanks to terencechan in the forums for the tip!