I don't think time determines anything on this classification. If ANY of your current gains are from drug use, I'd say you're clean. If you go from A-B without drugs, I'd say natural.
Guy 1: Juices, puts on 20lbs, keeps 10lbs...2 years later, maybe puts on another 5lbs....this guy would be clean.
Guy2: Juices, puts on 20lbs, loses 30lbs after and stops training for a few years. Starts up, puts on 20lbs himself. I'd consider him natural.
Turns out, I'm more like guy #2.
What matters is how you feel about your own gains, regardless of how you got them. If you take pride in taking drugs and putting on 10 easy pounds, you expect less from yourself than I do...that's all.