Hi,
The Basquin law gives a relationship between the number of cycles N and strain S. The product of the both terms is constant. Actually not exactly the product of N by S, but rather N*S^b=Constant. In this equation, b is a very important coefficient. Practicaly, b is fixed between 5 and 8.
Now, the reduction rule of the test duration is :
reduced acceleration = real acceleration * (Real time/ Reduced time)^(1/b)
That's it!