A conditional discharge becomes spent as soon as the period of that discharge has ended (subject to a minimum period of 12 months). That means that you don't have to declare it when applying for employment (other than where that employment is exempted from the provisions of the Rehabilitation of Offenders Act) or for things like insurance applications.
However a criminal record is for life and (as things stand at the moment) all convictions will always show up if a CRB check is carried out (whether at 'standard' or 'enhanced' level). There have been some suggestions that the CRB might eventually introduce a 'basic' level check (where only unspent convictions will be shown) but nothing concrete seems to be happening yet.
Offences which don't result in convictions can be omitted from CRB checks (if they're dealt with by fixed penalty notices) or removed after a certain period of time (if they're dealt with by a caution). But convictions remain on a person's record throughout their lifetime, even if the penalty is only a discharge (either absolute or conditional).
However (with very few exceptions, such as those for people with recent convictions seeking to acquire SIA licences to work as nightclub 'bouncers') a criminal record does not present an automatic bar to employment. It's entirely up to the employer's discretion as to whether they'll regard the conviction(s) as relevant.
Chris