Theoretically, cautions can never become spent. That's because a caution is not a conviction and, under the Rehabilition of Offenders Act, it's only convictions which can become spent.
When the system of cautions was first introduced, one of its key points was that a caution would not create (or add to) a criminal record. Unfortunately nobody, at that time, had bothered to define exactly what was meant by a 'criminal record'. When the Criminal Records Bureau was set up, its brief was to record both convictions and cautions, which effectively undid the good intentions of those who had introduced the system of cautioning. It also left cautions in a 'grey area' because the Rehabilitation of Offenders Act made no reference to them.
However, while there's still no official provision for cautions to become spent (in the same way that most convictions can), common sense seems finally to have prevailed and there's now an acceptance that cautions informally become spent after the same period of time as most non-custodial sentences do. (i.e. after 2� years for those aged under 18 at the date of conviction, and after 5 years for those who're adults upon the date of conviction).
The CRB has also finally recognised that cautions are not meant to fall into the formal definition of a criminal record. So, unlike convictions, a caution won't show up on a standard level CRB check after 5 years. However, an enhanced level CRB check can show any information (including cautions and even entirely unsubstantiated allegations made against a person) that a senior police officer, or other relevant authority, deems to be relevant.
Chris