Cautions are now covered by the Rehabilitation of Offenders Act 1974 so will become spent immediately (apart from conditional cautions which will become spent after 3 months). This means that if you are asked on an application form if you have a caution you can reply 'no'. For conditional cautions it would be after 3 months since the caution was issued, up until that time you would have to reply 'yes'.
This applies retrospectively so applies to anyone who has ever had a caution, irrelevant of when it was given.
However, this does not mean that it will not be disclosed on a CRB check. It will not normally be disclosed on a standard check but maybe disclosed on an enhanced check and on a subject access request - see below for more information.
Depending on the type of offence cautions are 'stepped down' after a set period of time (5 or 10 years). This basically means that they will only be visible to police staff and not other agencies who do checks. The exception to this is if a person is the subject of an Enhanced Disclosure and then if relevant it will be disclosed.
Even after five/ten years the caution can still be disclosed if you apply for certain types of jobs, i.e. police, teacher, or jobs working with children and vulnerable adults. It will be disclosed if it is relevant to the job you are applying for.