No rule of thumb. I try not to worry about shutter speeds with macro work.
Use a tripod and let the camera worry about how long to expose the scene. You need the DOF and you don't want to use ISO 800 so you just have to put up with longer exposures unless you are using a flash.
You cannot do true 1:1 macro work without a tripod. The depth of field is too shallow - the slightest movement of the camera will throw the subject out of focus - perhaps in full sun you can get the speed you need but who wants to take a photo in full sun!
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