There are always exceptions with particular audiences and scenarios, but the general rule seems to be the longer your form or survey, the less likely people are to complete it. And if they do they spend less time thinking about later questions.
An interesting bit of research by survey monkey looks at the average length of time survey participants spend on each page of a survey. The later in the survey, the less time spent considering the questions.
How Much Time are Respondents Willing to Spend on Your Survey? - surveymonkey.com blog
This ties in with form usability expert Caroline Jarrett's advice about keeping forms as short as possible and not asking unnecessary questions. Also the case study where altering a single field saved a company $12m.
Previous blog posts:
$12m - the cost of one extra field
Forms that work - book by Caroline Jarrett & Gerry Gaffney