As far as how long it has been since the Great Disaster, there are a few clues here and there. Kirby himself said in an interview (which is dubious at best cause he liked to fuck with people) was "less than a hundred years". However Part of Kirby's charm was how much he got away with logic-wise.
I would ad that I like the way the new "Kamandi's world" had the way the virus caused man to go down the evolutionary latter and the animals to go up it. Everyone kinda meets in the middle,