Generally, it takes 2-3 stinkers before I totally give up...and I may still borrow another 1 or 2 before I just give up reading the series altogether.
The Cornwell series ficbot was talking about had been going downhill IMHO for a few books before that last WTF action...and it's the last one I've read.
I got fed up with Brockmann's series about 8 books ago now, but I still pick them up if they're on the 100% micropay rebate at Fictionwise. I haven't decided if it's the writing or just that my tastes have changed.
On the other hand, I'm entirely frustrated with what Evanovich has done with her Stephanie Plum series...but I keep reading them FROM THE LIBRARY. I won't pay for it, but I just can't let it go.