There has never been a clear cut answer to this question: how much alcohol can be consumed before one is over the limit? It really varies from person to person. A petite woman may a have one glass of wine and be over the limit. A large, athletic male may be able to consume several drinks over a short period of time and still manage to be under the limit. One thing that should never be confused with the legal limit, however, is a person's tolerance level for alcohol. While this may come in handy on certain field sobriety tests, it certainly will have no impact on a breath test result.
In Massachusetts, the government can prove a person's guilt on an OUI charge in one of two ways:
1) by showing that the amount of alcohol that person consumed was enough to effect their ability to operate a motor vehicle safely or,
2) the "per se" rule - that the person's blood alcohol content was .08% or greater.
The government does not have to prove both of these theories to find someone guilty. They simply need to prove one "beyond a reasonable doubt" in order to obtain a conviction. The case is much easier for the government if there is a breath test reading of .08% or greater that is admissible. Under the "per se" rule, if the government is able to show that, then the person is guilty. If there is no breath test, then the case is much more difficult to prosecute. In these cases, the government has to rely on any field sobriety tests that were administered as well as the officer's observations.
The safe answer to the question posed above is to not have anything to drink before driving. However, if a person has consumed two or more drinks within an hour, it's a safe bet that any breath test would be close to the legal limit.
Most attorneys would agree that if you are stopped on a suspicion of drunk driving, the less you do as far as field sobriety testing, the better the case will be in court.