Almost immediately, if it's a strong sample.
Let me back up. The higher the concentration of alcohol, the longer it takes for the fuel cell to actually process all of the alcohol.
I don't propose to do what I'm going to do later tonight and lecture in the way that I'll be lecturing my fourth-year students. However, the process is that alcohol is collected and then broken down into component parts that produce electrons. The electrons are then changed to a voltage that is proportional to the amount of alcohol there. The larger the amount of alcohol, the longer it takes to do that, so a very high concentration takes a little bit longer to process than a lower concentration.
All that being said, it's a very rapid process.