Re: Depends upon the cataclysm involved . . .
What would take us down?
If the demise was benign to the planet itself (i.e., disease, abrupt ozone depletion), then it would require many billions of years for the Earth's crust to rotate our fossil-evidence back to the mantle -- barring a heavy impactor that could do the trick in a matter of days, hours, or even less.
I think anything that would effect the planet itself would require less time, again barring that impactor.
Dan