When you spray potatoes for blight, the spray (Dithane, Burgundy Mixture, etc) forms a physical barrier on the surface of the leaves and halums. This prevents the blight spores from attacking the plant and kills the blight. None of the bright spray is absorbed into the plant itself. The tubers are unaffected.
What might happen is run-off may soak through that soil and onto the tubers, but again, this will not be absorbed. When you give the spuds a good wash they’ll be grand.
Short answer, there is no time period.
As an aside... when you spray a weed killer, say Roundup, near an edible, say lettuce, there is a period of time you must wait before harvesting the lettuce. Theres a term for this which I cannot remember. The time period varies depending on weed killer.
Thus does not apply to potatoes and blight spray.