How long has 10.0.0 build 4209 been out for Ultras?
I was at 4195 last I knew months ago. Somewhere possibly between 4195 and 4209 I was having an issue where my Sony OLED was crashing every time I turned on. I had only Roku and a USB stick attached and I removed the USB stick and still happened so assumed was the roku but now it hasn’t happened in the last week and I’m 99.9% sure Android tv hasn’t updated since unlike Roku it asks you to update not forces you.
Roku Ultra 4660X software-version 11.0.0 software-build 4193 Roku Media Player 5.5.13
As @DBDukes indicated, rollouts are staggered in several different ways (user account/model #/location/device update server settings/mysterious Roku selection algorithm/etc).
E.g. 10.0.0-4209 may be available for download/install for some 4800s before other 4800s, or for the 4670 before the 4660 (or vice versa), relative to the user account/device settings/region/etc.
So theres no perfect way to answer that question, though in general build 4209 was first made available for download (for some models/devices) on June 22th, and then a wider release on June 28th.
You can always check when YOUR device last updated its firmware by checking (as @DBDukes indicated) Settings/System/System update/Last Updated =
FYI, if you manually check for updates, and a new firmware build is available, you do have the option of installing or not (you can decline to install it). Eventually the new firmware will auto-install, but there is an option to delay it by manually checking/declining.
Also FYI, Android TV devices by default auto-update both the firmware and the OS components/apps (especially when they power on/restart, just like Roku devices), so you might/could easily miss them (even if they do allow disabling of auto-updates, its enabled by default). For OS components/apps, you have to go into the Google Play Store app's settings and disable Auto-updates (otherwise they too update by default).