I want to dynamically kick out screeners

How can I create a variable “time limit” so that the survey detects and screens out speeders?

You can make that information available within the survey, by adding a ‘derived value’ field that picks up the time taken between any two pages.

2 page timestamps are recorded for every page. One when it is displayed and one when it is submitted.

For example - the timestamp for when Page 1 was displayed is the variables $PAGE_DISPLAY_1

So to calculate the time someone took between pages 1 and (say) 10 then decide to kick them out on page 11 if it was less than 60 seconds, then create athe expression is simply

Please note that the value will be measured in seconds, also you can of course only calculate this value on a page after the ones you want to measure.