Over the course of it's 12 years (1990 to 2002), I think several people had periods along the way where they personally "lost interest". Myself, I lost interest in the Fall of 1995, after the final new episodes of Fox Kid's version of "Batman" premiered, they lost the rights to Animaniacs, and they started airing stuff like "Goosebumps"....but I came back and watched Digimon from 2000 to 2002.
I think a better question is when it started to decline, some people say September 1999 following the changes, meanwhile internationally, Fox Kids was still expanding. September 2001 was the beginning of the end as that was when Fox Family Worldwide (along with Saban) was sold off, but even when that happened the block still ran its course for another year to wind down. 4Kids' Fox Box carried on where Fox Kids left off with the same principle of kid friendly anime, Fox just wasn't interested in running a kids block themselves anymore.