I’ve heard variations on this for years, but here is where I found this specific version: https://jessicadavidson.co.uk/2016/03/04/how-long-does-it-take-to-become-enlightened/

A Buddhist monk approached his teacher and asked the Zen Master, “If I meditate very diligently, how long will it take for me to become enlightened?”

The Master thought for a moment, and then replied, “Ten years.”

The student then said, “But what if I work very, very hard and really apply myself to learn fast. How long then?”

The Master replied, “Well, then it will take twenty years.”

“But if I really, really work at it. How long then?” persisted the student.

“Thirty years,” said the Master.

“But I don’t understand,” said the disappointed student. “Each time I say I will work harder, you say it will take me longer. Why do you say that?”

The Master replied, “When you have one eye on the goal, you can only have one eye on the path.”