You do not " hatch " fossils in Emerald, but you can revive them at the Devon Corporation on the second floor in Rustboro City by talking to a scientist there.
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i don't have Pokemon emerald, but the answer is always the same. not how long. how many steps. walk ALOT and the egg will slowly start to hatch
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You can't, you have to trade over from FireRed or LeafGreen.
level 20, as with all fossils in all games