It is a global setting, assuming AO is still running some of the original vBulletin code. I asked a while back for it to be changed to 30 seconds because it does get annoying sometimes (especially when I used to be on dial-up and couldn't stay on for long - 60 seconds meant I could only make about 20 posts a week) but obviously JP didn't do anything about it.

Personally, I think it would involve a lot of work to implement and wouldn't really bring about that much of a benefit. It's also useful to prevent accidental double-posting (another thing that I used to get on dial-up when connections timed out).