I have been prescibed, 0.5 mg xanax due to major panice attacks?

It doesn't seem to help much because I almost had a panic attack a few days ago, but was able to make it subside with breathing excercises. If I feel a panic attack coming on, should I take more than the prescribed 0.5 mg? Also, how long does it usually take to kick in and start helping? Any advice would be greatly appreciated. Thank you!