As Seatrout indicates, a criminal record is for life.
Convictions become spent, for most purposes (except, for example, for job applications working in, or alongside, the 'caring professions' or in the police force) after a specified period of time. This is determined by the sentence imposed by the court:
For a fine, or a community service order, the period is 5 years.
For imprisonment of 6 months or less, the period is 7 years.
For imprisonment above 6 months, up to 2.5 years, the period is 10 years.
(All of those figures are halved for someone under 18 years old at the date of conviction).
For longer periods of imprisonment, convictions never become spent.
Further information is here:
http://www.lawontheweb.co.uk/rehabact.htm
It should be remembered that the Rehabilitation of Offenders Act doesn't apply to situations covered by the laws of other countries. In particular, some countries (notably the USA) insist that anyone with a criminal conviction must obtain a visa before visiting that country. Convictions never become spent for the purposes of obtaining visa for other countries, so anyone (with a criminal conviction) wishing to visit the USA has to apply for a visa. (This takes 5 to 6 months and involves a lot of hassle and expense).
Chris