I think the answer to your query is this, taken from the site of an organisation in the US who do hair testing for alcohol:
“Approximately 200 strands of hair are required for testing. The hair samples will be 3 or 6cm long, depending on the time period your client needs to be tested for, e.g. 3cm of hair will cover a 3-month period. “
So you could look forward to driving open top (not a car, but your head!) if you have to do that for a year....!
Mind you there is an alternative to head hair:
“Head, arm, chest, and pubic hair can all be used for alcohol testing, however, the reliability and accuracy varies. Head hair is the preferred sample as it provides the most accurate picture of alcohol abuse. Arm and chest hair can determine excessive alcohol abuse but the time period is not as concise as head hair. It is not advised to use pubic hair as the test is designed to detect ethanol, and urine has a high concentration of ethanol, leading to false-positive results.”