Fig.: for an indefinitely long time; forever

When I told my father that I lend cousin Liz, who is always broke, some money, he said: "Nice going! You'll get that back when the cows come home!

Quando ho detto a mio padre che avevo prestato dei soldi a mia cugina Isa, che è sempre in bolletta, ha esclamato: "Bell'affare! Li rivedrai alle calende greche!"

I don't know why you cannot be one minute late in paying your taxes, but you can wait till the cows come home before you receive a refund

Chissà perché le tasse devi sempre pagarle perfettamente in tempo, ma i rimborsi arrivano sempre alle calende greche

There's nothing to do, we are politically too far apart. We could argue till the cows come home but I won't change my ideas and you won't change your views

Non c'è niente da fare, siamo troppo distanti ideologicamente. Potremmo stare qui a discutere all'infinito, ma io non cambierò le mie idee e tu non cambierai le tue convinzioni

Presumably the time referred to is when the cows return to the barn for milking at the end of the day. Cows are very unhurried animals and take their own sweet time at a leisurely pace to return home. The term has been around since the late 16th century