How do you decide when to say when?

How do you decide whether you like a series?  I know people who'll only give a show one episode (or just the opening theme song animation) to hook them or one volume of manga. 

For me, it seems like I'll give a manga three volumes to make me care.  If I don't like characters or plot after spending that much time with them, I pretty much don't feel bad about giving up. 

With anime, I don't know if it's because I do enough research ahead of time or because I have very few expectations going into a show or what, but I can't think of any series that I didn't watch through the end of what was available legally once I started it.  If I start it, I will finish it, if only to give it every last chance to redeem itself.  Not all of them do (Give me back those hours of my life, Glass Fleet!), but I'm usually somewhat satisfied and comfortable with them by the end.  I usually invest enough of my own energy into the show to get something back.  And it's true I'm easily entertained.

So again, what is your measurement of choice when deciding what is worth your manga-reading/anime-viewing time?