If 2 guys can use XNA to write a basic 3D RPG ( with full shader lighting ) in 4 days from scratch, I think it would be possible to create something basic and fun in eight hours if you are fully focused. If nothing else a basic prototype.

I'm sure you, Dirk, have created some usefull and probably excellent stuff for your games in 8 eight hours. Especially if you are *in the zone*.