It’s worth remembering that any background check only ever provides you with information on an individual at that particular point in time. At any subsequent point, an employee’s circumstances could change, meaning they may subsequently fail to meet the pre-employment screening criteria you had in place when they joined.
They may have picked up a criminal record, a CCJ or been declared bankrupt. They may have accrued points on their driving license or even lost their license. They may be dealing with family issues, ill-health, debt or other highly personal issues which may put them under pressure and lead them to act inappropriately. Alternatively, they may even have set up a separate business of their own, which could conflict with, or detract attention away from, their role within your business.
This is why regular, ongoing monitoring of existing employees is becoming more popular. Rather than thinking about how long a check may remain valid for, consider how long you would want to go without re-checking an existing employee.