Quotations about:
    now or never

He that will not when he may,
When he will he shall have nay.

Robert Burton (1577-1640) English scholar

Anatomy of Melancholy, Part 3, sec. 2, member 5, subsec. 5 (1621, 2nd ed.)
    (Source)

Generally attributed to Burton, but the text is set as a quotation on the page. This form seems to date from c. 1450; an earlier 1303 passage goes:

He that wyl nat when he may,
He shal nat, when he wyl.

More discussion here and here.