I voted Medium... but this rule only really applies to certain types of fiction for me. If it's a really good series, but quite small (i.e. Dark Song, Tales of the Otori) I will still read it... but if it's huge, say ten or eleven books... my interest wanes a bit near the middle (especially if you are forever waiting for the next book, as I'm doing for a series at the moment.) Stand alone books, I think, are good up until a certain point - if it's a stand alone that should be continued into a series (i.e. Parkland), I get disappointed and put off of those types of books for a while. But then there are, like, stand alones that are somehow related to another series which can be good... but sometimes they can actually ruin your perception of the series... yeah, I tend to veer away from those unless the series is really good.
And there are also these sort of "non-chronological" serials.. where they don't really run in a set order, or they don't follow the same story, but are somehow collected together at some point and called a series - sort of like a collection of short stories except there's one story per book... which also leaves me wondering whether we call a stand-alone book of short stories a "series" or a "singular" book...? The funny thing, for me, is that these sort of expectations and regulations are really only applied to the sort of fiction I read. Non-fiction is another story.