So, if you took 5mg Xanax, 12-15 hours later, your body would excrete
2.5mg. 12-15 hours later, 1/2 of the
2.5mg would be excreted and so on until it was completely out of your body.
Depending on why you need this information, you're actually lucky. Xanax has a notorious short half life. There are many meds out there that have half lives of days, even weeks!