120 generations — from now back to Plato and Socrates
500 generations — from now back to the invention of writing, agriculture and the first cities
80,000 generations — from now back to the Plestocene (the dawn of Homo Sapiens).
I found this in Denis Dutton's The Art Instinct (pg 23-24). He uses it to explain why evolutionary psychologists (and Komar & Melamid) have discovered that every human culture, everywhere in the whole world, finds the same type of painting most pleasing: landscape.