When a ewe lambs, the opening and stimulation of the vagina preps her for receiving the olfactory stimulus of the lamb so this can be manipulated for a few days afterwards by wetting the foster lamb, lying the ewe down and stimulating her vulva. Obviously the longer from actual lambing it is left the less likely that this will work, but 3-4 days is normally the max I have found. However it is normally recommended to keep the ewe and the new foster lamb in a small pen together on their own for a few days afterwards so that they can bond with no distractions and means you can check that everything is going ok.
It does also depend on the breed and the temperament of the ewe as some don't even like their own lambs! To stop the ewe drying up too quickly you can hand milk her which will stimulate more milk to be produced as it would be as if a lamb is suckling.
good luck!