As long as there are detectable amounts of alcohol in your body, a breathalyser test can show up as positive. The alcohol interlock device uses sensitive fuel-cell technology to measure the alcohol in your body. Even if you feel alert and functioning, there is a likelihood that your body has not finished metabolising the alcohol you drank, which will be reflected in BAC test results. Depending on how much you drank, often this extends to the morning after.
So how long?
Alcohol leaves the body at about 1/2 - 1 standard drink per hour. So if you drink 5 standard drinks, taking between 5-10 hours to sober up is a reasonable estimate. But keep in mind that we rarely know with exact certainty how much alcohol we're consuming. Bars and restaurants often serve in glasses larger than a standard drink. Different types and brands of alcohol can have vastly different alcohol contents. Each person metabolises alcohol differently as well, and that rate can change depending on the day, or with ageing.
If you want to know in even more detail, consider buying a good quality fuel-cell breathalyser, such as the Ajen AL001. A regularly calibrated fuel-cell breathalyser will give you information on how fast your BAC goes down after drinking. Only get behind the wheel if adequate time has passed since your last drink, you are alert, and your calibrated, fuel-cell breathalyser records two BAC readings of 0.0 ten minutes apart. This is not a hard or infallible rule. Use common sense and caution as well.