One thing I like about challenges (especially on RBY) is that the 'fun' in the game for me is to complete the challenge. If I'm just playing Pokemon normally, the fun is to train my Pokemon, grow with them, and catch as many as I can. Since I don't have to worry about that, it means I can run at 5000% speed most of the time. So I don't have time to get bored of the challenge, because I've already beaten it by the time I start getting bored. I mean, take my history of challenges. I've now done all 15 monotypes in RBY, about 1/4 now in GSC (I just beat Normal before I started posting today), a few in RSE, and 2 in DPPt. I've beaten like 7 solos in RBY, and none in any of the other gens. Also for all of my ultimate challenges, I've almost always finished the first gen, but not the others. Its because the others are longer, and that's about how long it takes me to get bored.
In other news, on to Grass on GSC (I think)