Adblock has been in the chrome store forever and Adblock blocks google ads across the interweb and not just on gmail.

Do you really think that ad impressions are the driving force behind gmail?

I doubt they will, seeing as how they've never blocked this: https://chrome.google.com/webstore/detail/gighmmpiobklfepjoc...

If you ask the question in the light of "how many people use this, compared to active Gmail users?" (11,616 vs ~ 200 million [1] = .006%), and then ask "how much is it worth to block this?" (probably a very small number), I personally come up with "a very very long time, if ever" for your "how long until..." question.

[1]: http://www.quora.com/Gmail/How-many-total-active-Gmail-users...