How long does it take untreated cholesteatoma to get to the stage where it erodes through the skull into the brain?

Several years: It really depends on what part of the ear (mastoid) bone the cholesteatoma is in, but you shouldn't wait for that to happen. The growth can do much more damage to the ear structures before it erodes into the brain. That can lead to permanent hearing loss and balance problems for a start. If you have been diagnosed with a cholesteatma it should be removed before any major complications arises.