A criminal record is for life. If a potential employer carries out a CRB check it will always show up. Similarly, if he appears before a court in the future, for sentencing in respect of another offence, the court will be made aware of his conviction.
However, most employers don't carry out CRB checks. The (ex-)offender can lawfully answer 'No' to any questions about whether he's got a criminal record once his conviction has become 'spent'. (The only exception is if he applied for work where the provisions of the Rehabilitation of Offenders Act don't apply, such as working with or alongside children or vulnerable adults). The length of time before a conviction becomes spent is determined by the sentence passed by the court. See here:
http://www.lawontheweb.co.uk/rehabact.htm
The only form of 'register' which exists is the Sex Offenders Register. By definition, only those convicted of offences of a sexual nature can be placed on that register. (That can lead to some apparent anomalies, as far as offences against children are concerned. For example, someone who pulls down a child's underwear and administers a playful smack can end up on the register because 'indecent assault' is a 'relevant offence'. Whereas someone who repeatedly lays into a child with his fists won't appear on the register because 'ABH' and 'GBH' are not 'relevant offences').
Chris