As Lawman states, a criminal record is for life and convictions will always show up if an employer carries out a CRB check.
As Bullseye states, most criminal convictions become 'spent' after a certain period of time. However, if the sentence exceeds 2� years (irrespective of how much time is actually served in prison), the conviction never becomes spent. So, if you're asked on an application form (whether for employment or for things like insurance) whether you've ever been convicted of a criminal offence, you'll always have to declare it (or risk being further convicted of 'attempting to obtain a pecuniary advantage by deception'):
http://www.lawontheweb.co.uk/rehabact.htm
Chris