I'm curious as to how fast does meth mouth occur?

Very fast: Just as meth speeds up the brain. It also speeds up bacterial acid production while diminishing protective saliva consistency.

That depends: Depending on diet, level of oral hygiene, and other health issues it can happen in as little as 4 months. For other addicts it can take up to a year. Meth is a very "ugly" drug and destroys the body on many different levels. If you are a user please seek help asap to end this destructive habit.