It depends upon the sentence:
If it was non-custodial, usually after 5 years. (There are shorter periods for some types of non-custodial sentence but those types of sentence, such as an absolute discharge, would not apply to GBH).
If it was a prison sentence not exceeding 6 months, after 7 years.
If it was a longer prison sentence not exceeding 2� years, after 10 years.
(All of those periods are halved if the offender was under 18 on the date of conviction).
If the sentence exceeded 2� years imprisonment, the conviction will never be spent.
There are some types of employment applications (e.g. working with children or vulnerable adults) where no convictions are ever regarded as spent. Also, convictions are never regarded as spent when foreign laws apply (e.g. when applying for visas to visit other countries). All convictions, including those which are spent, show up on a CRB check.
Chris