Every New Moon/Full Moon would make sense. If that's too low, then to every 24-72 hours. I know (or I hope) it's there to keep players from making easy money. But when impatient saps are giving up 25-100k because they want to be a Vampire/Werewolf right then instead of just waiting a while for a freebie, it defeats the purpose. I feel like there'd be far less people charging if the cool down wasn't so long. I myself have been tempted to charge or to keep the money grateful players donate to me after turning them simply because it's such a long wait and I can only do it once. Every day I get these lucrative offers when I'd be more than happy to do it for free, it's just that the ridiculous cool down makes me want to horde and hold on to it.
Rinnegan Dovahkiin: Daedric Prince of Balance