not until the cows come home

till the cows come home

For a very long, indefinite amount of time; forever. You can wait till the cows come home, but you'll never see that law passed in this country! My wife was talking on the phone with her sister till the cows came home last night!
See also: come, cow, home, till
Farlex Dictionary of Idioms.

till the cows come home

Also, when the cows come home. For a long time, as in You can keep asking till the cows come home, but you still may not go bungee-jumping. This term alludes to when the cows return to the barn for milking. [Late 1500s]
See also: come, cow, home, till
The American Heritage® Dictionary of Idioms by Christine Ammer.

till the cows come home

for an indefinitely long time. informal
See also: come, cow, home, till
Farlex Partner Idioms Dictionary

till the cows come home

Informal
For a very long time; indefinitely: They could argue till the cows come home and still not reach an agreement.
See also: come, cow, home, till
American Heritage® Dictionary of the English Language, Fifth Edition.

not until the cows come home

Not for a long time. Presumably the time referred to is when cows return to the barn for milking. The term has been around since the late sixteenth century. Beaumont and Fletcher’s play The Scornful Lady (1610) stated, “Kiss till the cow come home.”
See also: come, cow, home, not, until
The Dictionary of Clichés by Christine Ammer
See also:
  • til the cows come home
  • till the cows come home
  • till/until the cows come home
  • until the cows come home
  • doomsday
  • till/until kingdom come
  • till doomsday
  • till kingdom come
  • until doomsday
  • until kingdom come