Since the Bible Day era.
It was a running joke that I was a fan around 2004 and then one of my friends bought my the B'Day album and I absolutely loved it. The Beyoncé Experience DVD turned me in to a stan.
Britney: Since OIDIA
Beyoncé: Since CIL (I stanned for Destiny's Child since Bills Bills Bills, but I preferred Kelly over Beyonce in the DC days)
Rihanna: From the beginning, since I first heard PDR
Ke$ha - Tik Tok 2009
Kelly C - My Life Would Suck Without You 2009
Gaga - Poker Face 2009
Ellie - Halcyon 2012
Adam - For Your Entertainment 2009
Taylor - Fearless 2009