There is no such thing as Chromium browser, just packages of Chromium source built by different distributors like Ubuntu, Red Hat, etc. You might ask to be called back when a particular packaging other than Chrome gets significant share, but if you ask when Chromium (including all packagings) gets significant share, that's already happened.
The whole point of the "Chrome is proprietary" argument is that we don't know exactly what Google added to Chromium.
Since the diff between Chrome and Chromium is not open-source, there's no way to verify that Google only added what they claim to have added. For example, we wouldn't know if they slipped in an NSA backdoor along with a bunch of plugins.
There is no such thing as Chromium browser, just packages of Chromium source built by different distributors like Ubuntu, Red Hat, etc. You might ask to be called back when a particular packaging other than Chrome gets significant share, but if you ask when Chromium (including all packagings) gets significant share, that's already happened.