I agree with Reenie and will add:
I often quit surveys if it asks me irrelevant questions. So if a survey about iphone apps asks me my gender and marital status I will immediately quit. Equally, if a survey assumes information about me based on gender, age etc I will quit.
If gender is relevant, but I am given only a male/female option I will quit. The same if I'm asked my marital status and there is not a 'other' or 'no answer' box.
If a survey is poorly worded, difficult to understand, contains spelling and grammatical errors I will quit.
Finally, if a survey says "This will take 15 minutes" and 16 minutes in I have still not finished I will quit.
That's not to say that your survey has these issues, but just stating reasons why I quit surveys without finishing them.