Assuming gamma bursters effectively prevented any civilization galaxy-wide from emerging until around 65,000,000 BC, the first of those have by now each had sufficient opportunity to colonize the entire Milky Way galaxy

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CONTENTS of 59,999,999 BC- 51,000 BC Large land and aquatic mammals appear; many kinds of primates appear (almost as many go extinct); an island continent finally disappears for good; the Mediterranean valley turns into the Mediterranean Sea; human beings emerge, develop housing, clothes, lamps, and drugs, breed dogs, use horses; Mars dies (or goes dormant)

This page last updated on or about 10-31-05
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Approximately 15,000,000 BC: Assuming gamma bursters effectively prevented any civilization galaxy-wide from emerging until around 65,000,000 BC, the first of those have by now each had sufficient opportunity to colonize the entire Milky Way galaxy

Assuming virtually all planets galaxy-wide were prevented from developing higher lifeforms until approximately the same time the Earth did-- and that they did so on a similar schedule as the Earth, with the key differences being that they suffered no late mass extinction event around 65,000,000 BC like the Earth, and their highest lifeforms of the time (unlike Earth's dinosaurs) developed at a pace more akin to Earth's primates than its dinosaurs-- then the civilizations which emerged then have by now enjoyed 50 million years or so in which to colonize the whole galaxy.

At least fractional (10-20%) lightspeed propulsion for interstellar travel methods appear feasible to humanity, even at our present primitive level of technological know-how (2000 AD).
Thus, five million years would be a reasonable amount of time for a single star faring race to colonize the entire galaxy, even if equipped with only 10% lightspeed propulsion, and an average of 400 years was spent inbetween establishing a fresh colony and undertaking further colonization missions from that colony. If an interim period of 5000 years is substituted for the 400 number, then 50,000,000 years would be required to colonize the galaxy.
Possible resolutions of the Fermi Paradox due to things like aliens adhering to a Star Trek style "Prime Directive" demanding non-interference with primitives, or accidentally destroying themselves early in their history, or being disinterested in colonization altogether, might only work if the total number of emerging galactic civilizations is relatively small.
Just one star faring race with a history and motivations similar to our own, which avoided self-destruction, would be sufficient to colonize the entire galaxy no more than 50 million years after they began.

-- Scientific American: Feature Article: Where Are They?: July 2000 ["http://www.sciam.com/2000/0700issue/0700crawford.html"] by Ian Crawford

Apparently there are more challenges and obstacles involved in the successful galaxy-wide colonization efforts of an advanced star faring civilization than we will realize, circa 2000 AD; witness the lack of evidence that Earth was colonized (or even visited) by advanced aliens in 15,000,000 BC or later.

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