So... math. A one kilometer square map would have a maximum of 1000*1000*256 blocks in it (not sure what the 256 number's actual average per map would be but somewhere over 128, I suspect). A 2k map would have 4 times that. A 4k map would have 4 times the 2k. You get the pattern, I hope.
So you would have to potentially destroy 4 BILLION blocks to clear out an entire 4k map. Let's make it 2 billion to be closer to reality (discounting water and air blocks and other things like indestructible traders). Destroying one block per second would take over 34 million minutes (or just under 65 years) if I've done my math correctly. And that's just a 4k map.
It would be nice if they included a metric in the game that had the total number of blocks you've completely destroyed (and one for all the zombies, too).