(With apologies for the lateness of the reply):
As Jock says, convictions become 'spent' (for most purposes) after a period of time determined by the sentence for the offence.
It's likely that the time for 'd&d' would be 5 years but see here for precise information:
http://www.lawontheweb.co.uk/rehabact.htm
For some types of job applications, no offence is ever regarded as 'spent'. These are primarily jobs working in the 'administration of justice' or working with (or alongside) children or vulnerable adults. For such employment applications, your conviction will always have to be declared.
Convictions are also never spent when foreign laws apply (e.g. when applying for a visa to enter another country).
As Jock say, a criminal record is for life. However, he's incorrect when he states that it won't be revealed outside of the criminal justice system. If an employer (or potential employer) obtains a CRB check on you, your conviction will always show up.
Chris