now or never, it's
It must be done now or not at all, as in If you plan to state your case to the boss, it's now or never. This phrase was first recorded in 1560.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.