Imagine someone sends you an e-mail asking you to complete a survey. What is the biggest deterrent to your agreeing to fill it out?
Most likely, the biggest deterrent is the anticipated investment of time. If you feel that filling out the survey would take just a couple of minutes, you'll be much more likely to do it than if you think it will take ten minutes. Right?
Some market researchers offer incentives for people to fill out surveys. The incentive might be a coupon or entry in a drawing for a valuable prize. For many potential respondents, incentives do not overcome the deterrence of the anticipated investment of time. Moreover, if a survey is too long, some respondents will begin answering candidly but will eventually start to answer randomly to get through it as quickly as possible.
Not only are many surveys too long, but some survey solicitations don't tell potential respondents how long the survey is. If you want to minimize reluctance to complete your surveys, keep them short (no more than a dozen questions in many cases) and tell potential respondents how many questions are in the survey.