Never really. Use "is'nt" instead. "ain't" is not proper english and many people will correct you if you say it.

It is true that "ain't" is not acceptable in business or other formal settings. However, "ain't" is very much alive and widely used. It is used in informal situations and its use suggests that there is no need to be concerned about good manners, either because of the close relationship between the speaker and listener, or because soon one person will be punching the other person in the face. I grew up in the Southern part of the USA, and I associate "ain't" with uneducated people and more with people who work on farms, and not in the city. However, people with different life experience than me might also consider this word very "urban". Take a look at Wikipedia's entry: http://en.wikipedia.org/wiki/Ain%E2%80%99t

Ain't can be used in a variety of ways as a substitute for am not, have not, will not, won't, don't etc. I ain't going to study English anymore. I ain't got any money. (Sometimes you will hear "I ain't got no money" but it is a double negative which is really bad) I ain't sick. NEVER use ain't in any business situation as it is slang and would be considered bad taste.