As JudgeJ states, a criminal record is for life.
A conditional discharge becomes 'spent' after one year (or at the end of the period of the conditional discharge, if later). There is no need to declare it when applying for employment except for occupations which are exempt from the legislation.
JudgeJ correctly identifies jobs connected with 'the administration of justice' as among those where convictions don't become spent but probably the widest area of employment affected is working in, or alongside, the 'caring professions'. So, you will always have to declare the conviction if you apply for a job as a teacher, school caretaker, nurse, hospital porter, etc.
http://www.lawontheweb.co.uk/rehabact.htm
Chris