Most of the surveys can take anywhere from 3 minute to 60 minutes to complete. Average surveys could take between 15 to 25 minutes to complete. The approximate length of the survey will always be included in the survey invitation.
Keep in mind that you will be rewarded with points every time you have successfully complete a survey. If for any reasons you are not able to qualify for a survey, then you won´t get any points for it. More details here.