When determining the CBR value there will be two values. The CBR at the time of construction and then the long-term equilibrium value. When designing a carriageway both these values should be considered. If the CBR value at the time of construction is lower than the value at site investigation then a change in the foundation thickness may be required.
The CBR value is also very dependent on the moisture content of soil and can vary over time. The frequency of testing depends on the uniformity of soils, but the general recommendations are;
Uniform 0.5 to 1-mile intervals
Non-Uniform 0.25 to 0.5 mile
Highly variable 1000 ft to 0.25 mile
Testing standards will vary from council to council or company, so you should always ask their requirements for any testing carried out.