Idiomatic At this sole opportunity or time, or not at all.
[A]s you tender the welfare of your country, I entreat you not to forget or delay so public-spirited a work. Now or never is the time.1709 July 17 (Gregorian calendar), Jenny Distaff [pseudonym; Richard Steele], “Wednesday, July 7, 1709”, in The Tatler, number 38; republished in [Richard Steele], editor, The Tatler, […], London stereotype edition, volume I, London: I. Walker and Co.; […], 1822, →OCLC, page 232:
"Come off, now or never," cried Amyas, clutching him by the arm, and dragging him away like a child.1855, Charles Kingsley, “What Befel at La Guayra”, in Westward Ho!: Or, The Voyages and Adventures of Sir Amyas Leigh, Knight, […], volume II, Cambridge, Cambridgeshire: Macmillan & Co., →OCLC, page 309:
4 more examples
This was the moment when he must put his fortune to the test, to win or lose it all. Now or never must the balloon go up.1932 August 17, P[elham] G[renville] Wodehouse, chapter 13, in Hot Water, Woodstock; New York, N.Y.: The Overlook Press, published 2003, →ISBN, section 2, page 209:
It's now or never / Come hold me tight / Kiss me my darling / Be mine tonight / Tomorrow will be too late / It's now or never / My love won't wait1960 April 3 (date recorded), Wally Gold, Aaron Schroeder, Eduardo di Capua, “It’s Now or Never”, performed by Elvis Presley, New York, N.Y.: RCA Victor, published 5 July 1960, →OCLC:
Mamma mia, even if I say / "Bye-bye", leave me now or never / Mamma mia, it's a game we play / Bye-bye doesn't mean forever1975 March 12 (date recorded), Benny Andersson, Björn Ulvaeus, Stig Anderson, “Mamma Mia”, in ABBA, performed by ABBA, New York, N.Y.: Atlantic Records, published 21 April 1975, →OCLC:
There had been a sense of now or never about this semi-final for United. Another elimination would be a fifth last-four reverse under the manager and serious questions would be asked regarding their ability to break the psychological barrier that separates also-rans from winners.2021 April 29, Jamie Jackson, “Edinson Cavani and Bruno Fernandes help Manchester United hit Roma for six”, in Katharine Viner, editor, The Guardian, London: Guardian News & Media, →ISSN, →OCLC, archived from the original on 03 Oct 2023:
Of a need for a decision, an opportunity, etc.: arising at the present time and requiring action to be taken immediately, or not at all.
Margaret knew what it was to expect her visitors to return ‘in one moment,’ and with a ‘now-or-never’ feeling she began, ‘Ethel dear, wait,’ but Ethel was too impetuous to attend.1856, [Charlotte Mary Yonge], chapter XIV, in The Daisy Chain; or, Aspirations. A Family Chronicle. […], London: John W[illiam] Parker and Son, […], →OCLC, part I, page 133: