Voice of customer
Scroll triggers
Ask once somebody has read enough of the page to have a view about it.
A scroll trigger fires when somebody has moved a chosen percentage down the page. It is the best available proxy for engagement on a long page, and it filters out the visitors who bounced before they saw anything worth asking about.
The depth is set anywhere from one to a hundred per cent. Somewhere past halfway is the usual choice: far enough that the visitor has genuinely read something, not so far that only the small minority who reach the footer are ever asked.
It is the natural trigger for content and category pages, where there is no completion event to hang a survey on and time on page is a poor signal because it includes everybody who left a tab open.
Like every trigger it is one per survey. If the useful moment is genuinely a combination, the right move is usually to narrow the URL targeting instead, so the survey only exists on the pages where the single trigger means what you want it to.