It has actually taken over 13 billion years if we're being pedantic. But that's not the point.
What we need to consider is the level of complexity life will be able to sustain after our annihilation. How long did it take to get to humanity from the last time life was at that level of complexity? Much less than 4.2bn years, since we likely will not be reducing life to the level of one single living cell.
What we need to consider is the level of complexity life will be able to sustain after our annihilation. How long did it take to get to humanity from the last time life was at that level of complexity? Much less than 4.2bn years, since we likely will not be reducing life to the level of one single living cell.