Compare your fave, and their peers that came out a year or 2 around your fave on how their global sales are doing right now -
Example for my fave -
Global iTunes Artist Ranking -
222 Christina Aguilera
252 Jennifer Lopez
347 Britney Spears
N/a Jessica Simpson
N/a Mandy Moore
662 Destiny's Child
Go here, and do the same for your fave, and the peers of their time -
http://kworb.net/itunes/extended.html