7 year cycle
I always thought that music from the mid-1950s onwards roughly seemed to have a 6-8-year cycle in the UK:
1956 - Elvis Presley and rock (ok, Haley and Rock Around The Clock was 2 years earlier)
1963 - Beatles (which morphed into the later hippy stuff)
1971 - Glam - Bowie, Bolan etc.
1977 - Punk
1983 - Synth-based stuff and new romantics
1991 - Hip-hop etc. by which time I had pretty much lost interest in the charts due to old age, senility or getting married (I forget which)