Lunar cycle - New Moon. I spoke with the GMs and we've set the New Moon as every 28 days, with this first New Moon on d114. It can also happen if some fool uses Telepathy in an unwise fashion with her (and a few empathic manipulations with other powers could possibly do it.). With enough effort, I'd say it could be dragged out through more mundane means, but that would likely take work.

So, minimum every other month, possibly more frequently. It's internal to Samantha being half-demon. In the blood, if you will.