You have to find the busy hour, and so will need to at least measure in bins of quarter hours. http://en.wikipedia.org/wiki/Busy_hour_call_attempts may be a helpful analogy.
You need to exclude known public holidays and weekends, and collect a minimum of one month's data. Once you have identified the busy hour, you do not need to measure outside it. It usual to collect separate counts for light and heavy vehicles, and to count the lanes independently.