For me, it's not so much that I get sick of a fabric, it's that I'm not very good at imagining what all the fabrics will look like together once they're cut and sewn up, so once I get into a project I often end up realizing that maybe one fabric doesn't play nice with the others or two colors are too similar and too close together or etc etc etc. I am getting better and I frequently remind myself that as long as I enjoy the process, who cares if I change my mind along the way.
I've been focusing more on scrappies lately, so I haven't been having that problem, thank goodness, and besides, changing my mind just adds more fabric to my stash.