The ten year ruler is a rough estimation of the amount of time a style will remain popular.
The rule states that what is popular will change roughly every decade, give or take a year or two.
One important thing to note about the rule is that the style in question is always what is referred to as the "counterculture". The prime fashion will always be the preppy brands of clothing. It's what attempts to rival that which truly changes.
Most of the past decades have followed the ten year rule. The most accurate timeline starts with the 60s.
60s.- The prime of the "hippie movement" (counterculture). Recreational drugs and music festivals, among most other things associated with the hippies was popular.
70s.- Bell bottoms, a new breed of hippie (less active in political activism and movements).
80s.- Big hair, tight pants, leather. Hair bands and power ballads ruled the airwaves.
90s.- Grunge. Dirty, unkempt, rugged, rude.
2000- Emos. Tight clothes, black mid length hair, emo bands.