How long before hydrochlorothiazide works? My blood pressure was just a little high, so my doctor put me on just hydrochlorothiazide to see if it will work without other blood pressure drugs. How long before my blood pressure will go down?

Usually within days. Within a few days you should start to see an effect -- but, hctz, hydrochlorothiazide, is not a strong drug. Your best way to lower BP is to not exceed 1500 mg of salt per day and to exercise 30 minutes daily assuming your doctor is ok with these recommendations.
Each. Each response is very individual but I usually like to wait at least several weeks to a month to see how diuretics affect blood pressure, sometimes longer. Typically each agent you use for blood pressure lowering will lower your BP by about 10 points or so. Good BP control is a long term project, and the goal is to prevent events (stroke, heart attack) way off in the future.