Generally speaking, an email open rate of 15-20% is considered "good." However, not everyone who will open your email will participate in your survey. Therefore, you can expect the percentage of subscribers who respond to the survey to be even less than that.
The best way to gauge the success of your surveys is to monitor the reports and watch for trends. If you notice one survey had more respondents than other surveys, determine the differing factors and replicate them in future surveys to achieve similar results. Take into consideration a few suggestions to increase your survey respondents and pay attention to your reports to help improve your results over time.