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It won't take very long. There are different variables that depends on how long the motor will last. It could last a few seconds without any issues. There is residual oil on the cylinder walls and bearings. I would add oil as soon as you can though.
That shouldn't have seized the engine but it's possible. Can you turn the engine over at all? This engine has such tight tolerances there is no way to be sure. I saw one run for 45seconds with no oil and be perfectly fine.