Just enough water to completely immerse the root ball. You might get your hands in there and 'work' the root ball a little bit to break up the dry clods of soil, too. Once potting soil gets really dry it's pretty hard to wet it again without some agitation.
You might check it before you go to bed, and if it's nice and saturated, (heavy when you lift it) then you can slip it back in the pot. You don't have to soak it over night. Too much water is just as bad as too little. The roots drown without air . . .
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