For the first three years a new employee gets a step increase. Then from 4-15 years every two years. After 15 years its every three years until you hit the 10th step then all you get is a cost of living increase depending on where you live in the US.
I have worked for the VA for 6 years and the only drug screen I have done was as a new hire. As an RN I am subject to random drug screens, but have never been asked to provide one. Your job title may or may not require random drug screens.