I got the below approach from a casebook: Work out how many people are in the store at a given moment, then estimate average shopping time and use that estimate how many shopping visits are made during a week, then estimate total regular customers.
Specifically though, how would it work? Say we have 100 people in the store at the moment, and average shopping time is 1 hour, does that mean we have 100 people * 8h / day * 7 days in a week = 5600 regular customers?