How long does it take for the meds to work?

I'm working with two people who both got prescriptions for Xanax as a PRN. My clients aren't able to communicate very well about how they feel or when they have anxiety, or whether or not it helps them, so we have to watch for behavioral signs. It would help me to know how people who are able to manage their own meds use this so that I can help my clients. I know that both of them have a lot of anxiety, although they might not even know what the word "anxiety" means. They express it by refusing to do something (like getting on the bus), or by yelling, running and hiding in a bedroom or bathroom, throwing things, etc. I can tell that they are afraid of something, even though there isn't usually anything that we can see as a threat to them, and their behavior often puts them at risk.

If you take a PRN, how do you know when you need it? Is there any sign that someone else might be able to recognize as a sign that you need your PRN?

Thanks!