A criminal record is for life. A conviction for ABH will always show up on any CRB check and, if you're convicted of another offence, it will be made known to the magistrates (or judge) when sentencing is being considered for that offence.
However when applying for most types of employment (and for things like insurance) convictions become 'spent' after a period of time which is defined by the sentence imposed by the court. If you were simply fined or given a community service order, the relevant period is 5 years. If you were given a prison sentence not exceeding 6 months, it's 7 years. If you were given a longer prison sentence, not exceeding 2½ years, the rehabilitation period is 10 years. (For sentences over 2½ years convictions never become spent).
If you were under 18 years when sentenced for ABH, all of the above rehabilitation periods are halved.
However no conviction can become spent if another conviction is not yet spent. For example, if someone was fined for ABH in 2008 their conviction would normally become spent in 2013. But if they were fined again in 2012 for another offence, the ABH conviction wouldn't become spent until 2017 (when the second conviction became spent).
Finally, when applying for certain types of employment (such as those working with, or alongside children or vulnerable adults) the provisions of the Rehabilitation of Offenders Act don't apply and convictions are never regarded as spent.
http://www.lawontheweb.co.uk/rehabact.htm
Chris