The limit, legally, is pretty cut and dry: commute time is not paid, anything on site is. If you have to wait in line for security, that's paid. If it takes 10 minutes to count out your till at the end of a cashier shift, that's paid. If you're sent to another location in the middle of the day, time getting there from your normal place of work is paid.
There are a small handful of edge cases that vary by state (e.g., if you have to change into a uniform when you get to work, that's considered on-the-clock in California, but not federally), but not many.
There are a small handful of edge cases that vary by state (e.g., if you have to change into a uniform when you get to work, that's considered on-the-clock in California, but not federally), but not many.