As Norman states, a criminal record is for life.
For most purposes, the majority of convictions become 'spent' after a certain period of time, so that they don't have to be declared when applying for insurance and for employment. (Even if a question is worded as "Have you ever been convicted . . .?" you can lawfully answer 'No').
However, there are some circumstances where convictions are never regarded as spent. These are principally situations covered by foreign law (e.g. applying for a visa to visit the USA) and employment applications for working with children, with vulnerable adults or in jobs associated with the criminal justice system.
The length of time it takes for a conviction to become spent is determined by the sentence imposed by the court (rather than by the nature of the offence). Full details can be found here:
http://www.lawontheweb.co.uk/rehabact.htm
Chris