If it is possible for a bacteria to survive being off the planet and to stay alive within a salt chunk for 250m years, then in a sort of “reverse-exogenesis” it may be possible that earth’s own microbes are already out there.

Or you could try 100 ma:

After a mere 68 days—an imperceptible sliver of time in the microbes’ geological timescale of 100 ma—certain types of microbes increased their numbers by 4 orders of magnitude. Over 99% of the microbes could revive. They must be sitting there over geological time—just waiting for some nicer conditions. Finally, they get a chance to revive. It provides some crucial information for understanding the habitability of life on Earth and elsewhere. Either the individual cells are somehow surviving for “ridiculous lengths of time” or they “are reproducing with less energy than we thought possible”. But one way or another “they are starvation artists”.

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