For example:
1. If it's a 'thank you' page after the payment process in an e-commerce site, then you should display the survey immediately - the visitor can read the page in a second.
2. If it's a blog article that takes 30seconds to read, you might want to wait for 20seconds before displaying the survey to allow visitors a chance to start reading the material.
3. If it's a general information page within your website and is likely to be one of several browsed during a session, then a delay of two or three seconds will ensure a visitor can start to engage with the material.