Terraria – In hardmode, how large of a gap can hallow/corruption cross

From the 1.0.5 change log

Mud and ash now have a chance to fall like sand when struck.

Playing with it currently I seem to get a mud slide about 1 in 4 mud blocks dug, as the chance is presumably calculated for all neighbouring blocks the actual chance per block is likely lower

As for mining, make a ceiling out of stone or dirt and dig under it, digging down should be easy, be careful when digging sideways to dig at your farthest reach.

for mud you can also plant jungle/mushrooms in it which seems to prevent the falling chance, this would be quite situational as normally if you are tunnelling through it you will be removing the grass/fungi too

EDIT

from the 1.0.6 changelog

Ash and Mud no longer fall due to gravity.

so ash/mud physics is now of interest only as an historic note (unless it comes back again)

With the Clentaminator present in the game, this is actually quite easy, if a bit time consuming and expensive. Dig hellevators every 120 blocks across the area you want to purify. With the Green Solution equipped, float down the each hellevator twice, first spraying to the left, then to the right. I recently did this in one of my worlds, and it worked perfectly. I'd recommend checking every so often after you're done to make sure you didn't miss anything. This will also turn any underground mushroom biome to jungle, so you may want to go back when you're done and convert it back with Dark Blue Solution.

When I did this, I left large swathes of land from the surface to the underworld converted, and even brought in Crimson from another world so I had all the biomes present. Just make sure that if you do this you put wider shafts (I left six blocks) around the corrupted/crimsoned/hallowed areas so they don't spread.