I have hit L6 after 7 years of steep upward trajectory. I have been hired with a PhD and four years of part-time industry experience. I got it now because two years ago our org got a new leader with a mindset very similar to mine, while at the same time a solution I've been developing for a couple years was just found by a tens-of-billions-usd problem.

Sure, over the 7 years I've met a couple engineers with a similar trajectory and one with a steeper one. But the vast majority will either take more time to get to L6, or plain decide to stay forever at 4 or 5.