It depends on the artist. I think most people here think for a big budget female pop star anything outside of top 10 is flop, and if its only in top 10 or a week its a flop.
janet jackson / kylie minogue / kate bush / prince
f(x) / grimes / missy elliott / shiina ringo / bjork / girls aloud
britney spears / beyonce / cristina / stacey q / azealia banks
ciara / ga in / of montreal / paula abdul / roisin murphy / the b-52s
george michael / 4minute / carly rae jepsen / cocteau twins / siouxsie and the banshees
pj harvey / annie / donna summer / chic / le1f / aqua / m.i.a. / neneh cherry / the human league