To be honest...I think the people who tend to lose interest are the ones who go "OMGZZZ Ayumi/Kumi/BoA/Namie LUV EM EVRY1 ELZ SUX" without actually listening to anyone else...so of course they're going to lose interest when they only like four artists
I've never lost interest. Whenever I feel like it's time for new music I just check out a new artist