it's now or never

said when you must do something immediately, especially because you will not get another chance

当你必须马上做某事的时候，尤其是因为你不会再有下一次机会的时候会使用这个俗语

机不可失;勿失良机

If you have to go to the bathroom before we get back on the road, go now, because it's now or never.

如果你想在我们上路前上厕所，现在就去，现在不去就没时候了。

This is your only chance, John. It's now or never.

这是你唯一的机会，约翰。机不可失。