From the first time that bitcoin passed 1000, it crashed more than 80% and then took years before passing 1000 again after the crash.

I think that if bitcoin crashes more than 50% and then takes more than 2 years to recover to previous highs, we will know it was a bubble. (That's nowhere near impossible now that bitcoin is past 7000. I don't see a crash to below 2000 as anywhere near impossible. A crash below 3500 is quite possible... and that's if bitcoin doesn't keep running. If it hits 10000 this year (as so many think possible, and in a bubble, anything is possible, it would only need to crash to 5000...)

But remember, it's not the crash that confirms it was a bubble. As you correctly mentioned, we see 20-40% pullbacks frequently, and 20% in the stock market would make it a confirmed bear market.
It's the shattered confidence and the resulting lingering for years before passing previous highs that proves it was a bubble.

Since bitcoin's bubble is tied to the easy money that is blowing up bubbles across other markets, it could keep running until everything pops, and then it will probably be years before it recovers as there won't be ready VC money next time it pops.