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It depends on the focal length of your lens and also which area of the sky you are imaging. Closer to equator means that longer exposure time is possible.
The simple way is to forget about formulas and just take a 10s exposure, zoom in and see if you see trailing. If there is no trailing try longer exposure. If there is trailing reduce exposure. In several shots you will have the answer.
Digital makes the life easy.
Scroll down in the thread mentioned above for some hints.
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