I don't see the harm in the limit, it generally takes me longer than 60 seconds to send out a PM. The only instance it takes less is copy/pasting the same thing to multiple people, but you can send to more than one person anyway.

If it's a genuine annoyance to some members, I also don't see the harm in lowering the time to 5 or 10 seconds... it'll prevent double-submitting the PM but allow for quick-fire messages.