There was a suggestion elsewhere in the thread of triggering the popover based on actions rather than time which might work better (Again, only on me that I know of - others will differ!)

For an example off the top of my head maybe the top of the bottom third of the currently visible window going past the start of the comment section[1] triggers it, at that point I'm probably done reading the article and about to leave or read some comments. There's a huge amount of variables here but if it was a good article and the popover is a mailing list signup chances are I'd be open to signing up at that point. From there you sell via email later on.

Unfortunately I don't have any real marketing data (or skills for that matter) to back me up which means there's a lot of "I" going on in my posts here, which definitely adds to the over-entitled jerk appearance. Chances are a lot of people will sign up and will click ads and websites would do a lot better with them as the reader than myself :-)

[1] Wow that was a horrible mash of words on reflection. Hopefully it's readable.