It's pretty subjective on how many times you might resend to unopens. It really comes down to how aggressive you want to be with whatever it is that you're sending. My take says that more than a couple times with the same message is probably too much, but if you're mixing up the content a bit, you can certainly pitch something more than that.
I would caution against bulk deleting folks... keep in mind that not all email clients will report opens accurately, so the statistic is an estimate at best. HEY, which launched last year, uses its tracking-blocking features as one of its selling points. If you delete those reporting as not opened, you could be deleting legitimate subscribers.
