How long after taking 0.5 mg of ativan can i have a beer?

Never. Alcohol is a poison that never did anybody any good, ever, so stay away from it. It will destroy your brain cells, facilitate development of cancer in many organs, including breast, cause hepatitis, cirrhosis and pancreatitis etc. There is no safe level of alcohol intake. Quit while you are ahead.

Not a good cocktail ever, agree with Dr Singh. If you suffer from an anxiety disorder try lifestyle changes first, benzos are habit forming and never a good long term solution. Talk to your prescriber as well if your symptoms are not controlled.