Poor context = poor UX
Yes, the pop-up looks like an advertisement. Yes, knee-jerk adverse reactions are to be expected. That is because most ads are not relevant and serve as a disruption/distraction: they are in poor context.
The optimal time is when it is relevant and beneficial to the user.
Showing an initially irrelevant screen is raising the barrier to entry. It is crucial to make a good impression the first few seconds of a visit. If you slap them with an AD, that is going to be the impression you leave. It is alot like speed dating.
Check out an article about banner blindness and how leveraging the users flow can result in a $300mm button.
One solution might be to leverage a checkout process. You have an engaged user, simply ask them to opt-in (or opt-out) in the checkout process. This way you have you lead who is already sold on the idea.
Another solution might be to place it on the page with a useful CTA.
While the simple answer is context, the exact answer will be test results.
Optimizely is a great tool for conducting multivariate tests.