“A. It depends on how much the person had to drink, and how long ago they drank. Our bodies eliminate alcohol at the constant rate of about one drink an hour. (More precisely, our bodies lower our alcohol level by .015% BAC per hour.) If a person?s intoxication level is about .02%, about one hour later their alcohol level would be zero. On the other hand, if a person had an alcohol level of .20%, twice the legal limit for drunk driving in most states, it would take over 13 hours for their alcohol level to reach zero after they stopped drinking. Once the alcohol level is zero, no alcohol test can detect how long does alcohol stay on your breath.”
This is, of course, an approximation. A lot depends on how much alcohol a person’s drinks and what his/her metabolic rate is–this differs in each individual.
This site said about the same thing with a little bit more info:
1. How long after drinking alcohol can we detect it with a breath alcohol tester?
It depends on how much the person had to drink, and how long ago they drank. Alcohol is eliminated at the constant rate of about .015% BAC per hour, which is about one drink an hour. If a person had only one drink, the maximum intoxication they might have would be about .02%. Within one hour, their alcohol level would be about zero. On the other hand, if a person had an alcohol level of .20%, twice the legal limit for drunk driving in most states, it would take over 13 hours for their alcohol level to reach zero after they stopped drinking. Breath alcohol testers measure blood alcohol level, which is a measure of actual intoxication. Therefore, a person who is not intoxicated has no alcohol in the blood, and a breath alcohol test will give a negative result. It seems that some people have the mistaken impression that alcohol stays in the system for a long time, as marijuana does. In fact, alcohol in the blood, and a breath alcohol test will give a negative result.
2. Will mouthwash interfere with the results of a breath alcohol test?
Proper test procedure requires that the subject have nothing in their mouth for 10 to 15 minutes prior to testing. If the subject has anything with alcohol in their mouth just prior to testing, the alcohol in their mouth will contaminate the test, giving an artificially high reading. However, 15 minutes is sufficient time for all mouth alcohol to be dissipated.
3. Are there ways to fool a drug test or breath alcohol test?
It is a little easier to fool a drug test than a breath alcohol test. Proper test procedure should eliminate the possibilities of a subject switching their urine specimen with another, or tampering with the specimen. Drinking large volumes of water will dilute the urine, thereby reducing the concentrations of drugs in the urine. The detection of some drugs, such as amphetamines, is affected by the pH of the urine with some kinds of methodologies. A properly administered breath alcohol test is difficult to fool. As long as the subject delivers an adequate deep lung breath, there is virtually no way to mask the alcohol. A subject who claims that their positive breath alcohol test is the result of mouthwash or cough syrup use may be asked to retake the breath alcohol test in 15 minutes. This is more than sufficient time for the remnants of alcohol in the mouth to dissipate. A second positive breath alcohol test after 15 minutes cannot be attributed to mouthwash or cough syrup.