Actually "frost line" is incorrect, you want it below the freeze depth of soil. How deep is that?I thought the frost line was 18 inches...
It varies widely depending on latitude, local topography, soil type, etc. ad nauseum. Some of the low-lying ground on our farm would only freeze about 12" deep, while some of the hills would freeze 2-3' deep.
Best would be to create a vault several feet under, put your protected stuff in that (see "three is a good number" above) and then cover it.