I think it simple extends XenForo_ControllerPublic_Conversation::actionInsert();
Find line from 590 to 593:
PHP:
if (!$conversationDw->hasErrors())
{
$this->assertNotFlooding('conversation');
}
Replace by:
Code:
if (!$conversationDw->hasErrors())
{
$this->assertNotFlooding('conversation', 3600);
}