As Norman indicates, a criminal record is for life but the majority of convictions become 'spent', for most purposes, after a certain period of time, which is determined by the sentence imposed. This is as follows:
For most non-custodial sentences: 5 years
For prison sentences up to (and including) 6 months: 7 years
For other prison sentences up to 2.5 years: 10 years
(Convictions are never spent if the sentence exceeds 2.5 years imprisonment)
For a more detailed reply, please see my answer here:
http://www.theanswerbank.co.uk/Law/Criminal/Qu estion438708.html
Chris