The duration of a test affects the sample size because the longer a test runs, the higher the number of transactions that can be executed. The type of test scenario will also affect your test’s duration. For instance, a soak test, which aims to measure how your application responds to a sustained load, will be insufficient if you run it for five minutes. The same five minutes, however, might be perfect for a spike test, where you want to test a high amount of transactions within a very short timeframe. In general, though, I would be wary about drawing conclusions from a peak load test that lasts less than thirty minutes.