It depends on precisely what sort of bad a fic is. If it's a decent plot or plot-premise but poorly written, or grammatically poor/obvious that English is not author's native tongue, I'll often stick it through to the end. If on the other hand it's a crappy/unappealing/cliched plot that's written skillfully, I'll often end up reading farther than I should but will ultimately leave off and search elsewhere. If it's just plain bad all around, poor plot and badly written, I'll abandon it pretty quick.

And do you know, it was you and your psyTunes that came instantly to mind when such a subject-appropriate song came up?

And yesss, dammit, you get the Gecko icon again. >_< It somehow always seems appropriate.