Random Opt in Pop Up Timing
In each situation, there is a different optimal time for an opt in pop up to be displayed. These are influenced by many parameters like who is the audience seeing this, how long is the content, what time of day, etc.
Have an option in your pop up management software to request a test where the pop-up will be displayed at random intervals. The more conversions you have for some of the times and the less for other the system will start showing the better ones more and drop the worst performing ones.