Apple said that it would be out in the fall. The earliest would be mid-September. The latest would be late October. Past that, it's anyone's guess.
The last time there was a massive top-to-bottom visual redesign of macOS (OS X Yosemite [10.10.x] in 2014), the OS was released in October. They may need additional time to rework that redesign. Unlike that redesign, this one also has massive under-the-hood changes coming as well (which justify this being macOS 11.0 instead of another release of 10.x), so it may be later in that range than it has been in recent years.
I would follow the news surrounding the developer and public betas (MacRumors, 9to5Mac, and AppleInsider are all good sources of Mac news and rumors) to get a clearer idea of how close Apple is to release. But, suffice it to say that when it comes out, you'll definitely see it available for upgrade either in the Mac App Store (if on macOS High Sierra or earlier) or via Software Update in System Preferences (if on macOS Mojave or MacOS Catalina).
If on Mojave and newer, once Software Update in System Preferences gets wind of Big Sur being available, you'll see a "1" badge on the System Preferences icon until you upgrade to Big Sur (as those on Mojave with the latest security updates currently see for Catalina).