Meaning
When a decisive point at which one's future course is determined.

Origin

This isn't a particularly old phrase. The first citation I can find of the phrase in that form is from The Times, July 1960:

"Even the holders of Government bonds turn out to be chiefly philanthropic institutions and trade unions when it comes to the crunch."

What is 'the crunch' exactly? Crunch isn't commonly used as a noun, but it seems that the word was taken up by Winston Churchill, who was fond of using it to describe challenges; for example, he was reported in The Daily Telegraph as saying in 1939:

"Whether Spain will be allowed to find its way back to sanity and health ... depends upon the general adjustment or outcome of the European crunch."

Of course, Churchill was a widely reported and influential author and speaker and his use of language was much imitated. The phrase when it comes to the crunch directly followed from his earlier mode of speech.