As others have indicated, a criminal record is for life.
However, under the Rehabilitation of Offenders Act, all convictions (except those resulting in a prison sentence over 2.5 years) become 'spent' after a certain period of time. A 12 month conditional discharge becomes spent at the end of the 12 months.
Once a conviction is spent, you don't have to declare it on applications for employment or insurance, etc. Even if a question is worded as 'Have you ever been convicted . . .?', you can legally answer 'No'.
The only exception (in the UK) to convictions becoming spent is when someone applies for a job which is exempted from the provisions of the Rehabilitation of Offenders Act. These are generally jobs involved with the 'administration of justice' (e.g. with the police or courts service) or in (or alongside) the 'caring professions' (such as teaching, school caretakers, nursing, hospital porters, etc).
As KittyWoo states, convictions will always show up if an employer carries out a check with the Criminal Records Bureau. However, the CRB will only provide the information if the type of employment is exempted from the Rehabilitation of Offenders Act because it involves close contact with children or vulnerable adults. (Other employers can't have CRB checks carried out).
It's also worth remembering that the Rehabilitation of Offenders Act only applies to the UK. So, for example, someone with a criminal conviction can never enter the USA without first obtaining a visa (which involves a lot of hassle and takes several months). The conviction prevents them entering under the 'Visa Waiver Program'.
Chris