临渴掘井 (lín kě jué jǐng): To act too late — Meaning & Stroke Order

Better to prepare in advance than to dig a well when thirsty.

He doesn't review regularly, and only crams before the exam—he still failed.

The company doesn't build a talent pool in normal times, and only scrambles to hire when a project comes—that's really not a wise move.

Thinking of buying insurance only after a fire breaks out—that's truly digging a well when thirsty.