I usually stick with something to a full season, or a full book, or the first full chapters of say, a webcomic. If it doesn't really grab me, I drop it. Soooo boredom is my petty reason? I try to give most media the benefit of the doubt. Sometimes it really does take a full season or two for a series to find it's footing. In the case of online comics, it could take a number of years! I really don't consider myself a completionist though. I know there's only so many hours in the day, and only so much time we have on this earth, so I don't commit myself to nearly as many series as I'd like. Then again... I have my own projects to work on
Also, I rarely get offended, and even then I may repeatedly go back to the offensive series before finally giving up on it altogether just out of sheer exhaustion (cough Family Guy cough). Because I am a masochist