I think we can reasonably conclude that Earth has not been assimilated at any point in its entire existence. If it had been assimilated in the distant past, it would not have continued to develop uninfluenced for the rest of its history, unless the AI's utility function were totally indifferent to our development. So we can extend the observed period over which we have not been assimilated back to a good four and a half billion years or so. The milky way is old enough that intelligent life could have developed well before our solar system ever formed, so we can consider that entire span to contain opportunities for assimilation comparable to those that exist today.

We could make a slightly weaker claim that no Strong AI has assimilated our portion of the galaxy since well before our solar system formed.