Destiny can be used once per time slot, or once during an event. They can be used a max of twice per day but that can be refreshed at GM notice. The exception to this is during Combat threads as those will be done in relative real time with back and forth between me and the combat participants.
I may raise this cap if people are under utilizing Destiny. For events I will have a list of Characters that I use Destiny against, one of their rolls will have a Difficulty die upgrades to a Challenge die. They can choose the roll. To help me out, list in the Destiny Points thread when you use a Destiny Point along with a link to the post so I can keep track.
If you have questions feel free to ask.