Duration is how long you want your flood to run, and it starts when your script actually begins execution, not when you launch your test. Grids generally take a few minutes to start, and this time is not included in the calculation for duration.
Duration affects how much a flood costs you because it is one of factors that affect how many VUH you consume. To keep costs down, we suggest running grids in multiples of 15 minutes. For your first shakeout test, we recommend running for 15 minutes or less.