“悬崖勒马”，汉语成语，意思是在高高的山崖边上勒住马（draw [rein] in one’s horse at the edge of a cliff）。比喻到了危险的边缘及时清醒回头（desist from doing something before it is too late），可以翻译为“pull back before it is too late”。
Now is the time for you to pull back before it is too late.
The criminals should stop on the precipice, retract from the wrong path and not go any further.
I had the chance of a lifetime, and I blew it.
It’s only once in a blue moon that you get an opportunity like that.