How do I make people fill out say, at least 25% of the survey, in order to submit?
Is there a general option somewhere in the survey settings for this? Or do I have to randomly go through and make certain questions required?
The simplest way to do this would be to make at least 25% of your questions required. The only other alternative would be to create a custom script. You could place a script on a hidden page at the end of your survey that counts the answered questions and asks the respondent to navigate back thru the survey to answer more questions. However, unless there is some serious motivation/incentive for the respondents to complete the survey I would assume that anyone who could not be bothered to complete at least 25% of the questions would simply quit.