I dug a 20x30 square hole that's about 30 blocks deep. The base of it is like 10 blocks off of the bedrock, so it is really more of a big empty cavern than a "hole" per se. Took quite a while, although not nearly as long as I thought it would. Digging like this doesn't yield enough iron to keep paying for the pick axes that you go through, you either need stone ones (which burn out really fast) or you need an iron stash previously prepared.
