It's now or never意思就是“机不可失，失不再来，勿失良机”。

【例句】
It's now or never, so make up your mind.
机不可失，下决心吧。

But if you did want to stop him, looks like it's now or never.
不过你要是想彻底阻止他，现在是最后的机会了。