Is there any reason for longer block periodes?

As I see it, either the spammer retry or not. So then even 1 second block would be fine.
Or am I missing something?

So to avoid mail delay I would like to set the block period in seconds rather than minutes.

Also what do SM do when a remote server respond with a "451 Greylisted, please try again in 10 seconds".
do you retry in 10 seconds or as defined in "Retry Intervals"?