[alexjplant]

You're confounding the investigation period for a clearance with what they'll accept as far as drug use. Generally speaking Secret clearances investigate back 7 years and Top Secret clearances go back 10. There are, however, questions on the SF-86 that are "ever" questions that ignore these timeline. Regardless you can have used drugs during these periods and tell them as such and there's a chance that they'll grant you the clearance. It's ultimately up to the people adjudicating the clearance and they use a reference guide that's periodically updated to determine this.

Years ago the rule of thumb was that they'd give you a clearance if it'd been 1 year since you used marijuana and 3 years since you used hallucinogens so long as you'd demonstrated a commitment to a drug-free lifestyle since. Anything serious like opioids or alcoholism would require you to have gone to rehab and seriously reformed your life.

In more recent times they seem to have gotten more lenient regarding recent marijuana usage, but I've been out of the industry for a minute so I can't say for sure. What I've always told people is that if you want a cleared job and have done drugs then 1) tell the truth, and if that would get your clearance application denied then 2) wait until it's been long enough so that it won't and clean your act up in the meantime.