[tor-mirrors] Mirror List Update Frequency

Please forgive the newbie question, but does anyone know how often the mirror list is updated? I am aware it's a manual process and I'm just curious because there are a lot of mirrors whose operators have sent removal requests and are still listed, or have sent update requests and are listed with incorrect information (about 20% are unreachable at the moment using the listed information), not to mention the ten or so awaiting testing and addition.