Directed soil sample points are shown in red. Grid sample points are in blue. The purple background shows the phosphate distribution.
On average, a grid soil sample will last four years, but that is not a black and white rule. A better answer to this question is, it depends.
The preferred approach is to select a few points from the original grid sample and monitor the nutrient levels at those points. Each season directed soil samples are pulled and compared to the previous samples. This shows how the low and high nutrient levels are responding to the variable rate applications. Which allows for adjustments to the fertilizer prescriptions to be made when needed.
As the seasons pass the original grid will gradually no longer represent how the fields nutrients are distributed. By monitoring with the directed points a crop consultant can determine when a field has changed enough to justify re-gridding.