The fact that life on any planet develops over such a long period of time, and is a key feature for detecting life via atmospheric biomarkers, should allow our search for life in our galaxy to be greatly reduced.
Our galaxy is only a hundred thousand light years across, so it doesn’t matter where we look in our galaxy, when we have the power to see atmospheric biomarkers on other planets, life on that planet can then be given an estimate to when life begun after the planet was created. All this can be determined simply by observation of it position and age, we could calculate which planets were most likely to have reached a stage of evolution similar to our own. We can begin to make a map of known planets with life at higher stages of evolution in our galaxy.
On our time scale, we should be able to see such details within the next two decades.