[movie English] expression: spinning what-if's and till the cows come home?

Dear All,

In "The Bridge of Spies" in the conversation between Tom Hanks and the judge, the judge says:

A: It is possible that in foreseeable future an American of equivalent rank might be captured by Soviet Russua. We might want to have some to trade.
B: Wow, that sounds like spinning what-ifs. You could do that till the cows come home.

- I assume that spinning what ifs is an expression meaning talking about possibilities that are just as likely to happen as not?

- till the cows come home - does it mean... forever?

Are these expressions popular? And do we say what-if's or what-ifs?

Thanks!