A criminal conviction stays on file forever. After a certain amount of time, many offences are considered to be spent and you may not have to tell an employer unless it is a notifiable occupation, for example a doctor.
If the offense/conviction occurred after your 18th birthday it will always appear on your criminal record. Crimes are not like traffic "points. They do not "go away" after a certain length of time. wiki.answers.com
Juvenile records - convictions prior to age 18 - are "sealed, meaning that they will not appear in most background checks and need not be revealed to prospective employers. If an individual is arrested and charges are dismissed, greatly reduced or www.ehow.com
The time can vary. If you are speaking of having your criminal record expunged of an offense - once the judge approves it (if he does) - court orders directing the expungement will go out to the various record-keeping agencies for them to act on it. wiki.answers.com