When it is safe to hire an ex-con?

After the offender has been clean for 5 years. Here's the press release:

When is it Safe To Hire Someone With a Criminal Record?
New Carnegie Mellon Study Provides Empirical Basis For Employers To Use in Assessment of Prior Criminal Records

PITTSBURGH—Carnegie Mellon University researchers have created a model for providing empirical evidence on when an ex-convict has been “clean” long enough to be considered “redeemed” for employment purposes.

The new study, which appears in the current issue of Criminology, estimates that after five years of staying clean an individual with a criminal record is of no greater risk of committing another crime than other individuals of the same age.

The study, funded by The National Institute of Justice, used criminal-history records of more than 88,000 first-time offenders in New York in 1980. Most committed new crimes within the first few years after their initial arrest, but only a small minority had a new arrest after staying clean for at least five years.

This is bound to be a fertile area for research. For example, the recent history of the American Catholic Church suggests that pedophilia has a high repeat rate. Episodes can be covered up for a long time, making the individual appear "clean".