A caution is a conviction. never except them, they are only given when the CPS wouldn't bother to prosecute. Also they help boost the polices percentages of convictions. As it is a conviction it will show up on a disclosure for a given number of years, whatever they say it will last for.
I work in a certain part of the security industry where a conviction including a caution will mean you can no longer work, usually for 3 years.