There is no set time. It is all dependent on volume and temperature. You're going to have to shake the keg periodically and listen for the ice crystals.
Beer, if drank with moderation, softens the temper, cheers the spirit, and promotes health.
"Give me a woman who truly loves beer and I will conquer the world." - Kaiser Wilhelm
"I would kill everyone in this room for a drop of sweet beer."- Homer Simpson