What happens if PayPal's IPN service cannot contact the IPN destination?

Please see details:

Is PayPal resending the IPN post request when the host is unavailable? Or is it simply discarded, and up to the administrator to deal with this?

Yes. PayPal will continue to send you IPN message if your server is not acknowledging the POST message or if your server is unreachable.

If the notification is resent, what would the delay be?

According to PayPal document, PayPal will re-send the message periodically until you responds, However the interval between retries increases with each attempt. An IPN will be resent for up to four days, with a maximum of 15 retries. The interval will increase after each fail attempt.