It depends on which governing body your employer is covered by. The following is a list of the time frames: FMCSA 3 yrs., PHMSA 2 yrs., USCG 2 yrs., FAA 5yrs. for pilots and 2 yrs. for others, FRA 2 yrs. and FTA 2 yrs. For companies not covered by federal regulations the time frame can be much longer. I have seen numerous situations where violations of previous companies drug and alcohol policies have shown up for ten previous years. It is advisiable to address any violation and get a substance abuse evaluation.