I'm sure we've all endured 2 years of on / off EastEnders or Neighbours due to our family or flatmate's viewing habits, but I was thinking more of the TV shows you've watched repeatedly and then given up on overnight.
For me, it's probably Mad Men. As a designer, had everyone I know raving about it for years so eventually just bought the box set. Wife and I got halfway through series 4 when one night I said "Shall we stop watching this? Its going nowhere" and she said words to the effect of "Thank Fuck. I thought you liked it..."
Didn't hate it and liked a few character arcs, but it in no way represented a decent return on the time I invested in it.
You? I expect to hear the word 'Lost' a lot...