Relatively modern amplifications of this one include "till the cows come home in the morning" and "till hell freezes over and the cows come skating home over the ice". The expression has meant "a long, long time" for a long, long time, since about 1600, and the idea behind it is that cows take their own good time about coming home if they aren’t driven, often until the next morning when, with udders painfully swolen, they come home to be milked.
Robert Hendrickson, The encyclopedia of word and phrase origins
Among the French equivalents I found following a quick research in my dictionaries and on the Internet: jusqu’à la Saint-Glinglin (until Saint Glinglin’s day), jusqu’à perpète (until perpetuity), jusqu’à la semaine des quatre jeudis (until the 4-Thursday week), quand les poules auront les dents (when chickens have teeth). However, I’m not sure they are absolutely appropriate. While the simplest translation for "til the cows come home" would be "for a long time", these French expressions mean "forever". In fact, "till hell freezes over and the cows come skating home over the ice" (which isn’t about to happen) seems to me to corrupt the original meaning of the "til the cows come home" (which will happen, sooner or later). Any thoughts?