This interesting study by Lightspeed Research looked at the relationship between the online survey length and quality of measurement and participation. In summary, study found that longer surveys result in higher drop out rates, and less reliable measures of constructs of interest. Interestingly, there was relationship between certain demographic and probability of dropping out, although that did not necessarily affect the representativeness of the overall sample:
"Respondents who drop out are somewhat different than those who complete the survey. Demographically, they are likely to be older, retired, male and from smaller households without children. In addition, those who drop out are less likely to do all the grocery shopping and tend to purchase snack foods less often. Despite their differences, the dropouts are so few in number that their exclusion does not harm the representativeness of the sample."
The study also found that, possibly, due to the fatigue, the quality of certain measure was affected by the survey length:
"In the Lightspeed Research study, the concept scores for the 8-minute questionnaire tend to be higher than scores for the longer questionnaires. The 20- and 24-minute questionnaires receive fewer “definitely would buy” ratings compared to the 8-minute version. There are also several notable differences on liking, value, uniqueness, and believability between the 8-minute version and the longer versions (see Table 2). Also, several important differences occur between the 8-minute questionnaire and the two versions with the extra upfront questions."
The position of questions pertaining to the main concept in the survey relatively to other attitudinal and demographic questions may also result in biased and poor quality responses.