A criminal record is for life. If you're convicted of any offence in future the court will be made aware of your previous conviction. Similarly your conviction will show always up on any CRB check.
However when applying for most types of employment (as well as for things like insurance), the provisions of the Rehabilitation of Offenders Act apply. That means that, after a specific period of time (defined by the sentence which was imposed for the offence) convictions become 'spent' and you can lawfully answer 'No' if you're asked if you've ever been convicted of a criminal offence.
There are certain types of employment applications where the provisions of the Rehabilitation of Offenders Act don't apply. (They are mainly for jobs working with, or alongside, children or vulnerable adults, as well as jobs concerned with national security or the administration of justice). For those types of employment applications convictions never become 'spent' and must always be declared.
http://www.lawontheweb.co.uk/rehabact.htm
Chris