Why one minute is the standard
The one-minute test is the most widely used benchmark because it is long enough to average out a lucky or unlucky start, but short enough to repeat often. Most published typing speed statistics — including the averages quoted by employers — are based on tests of roughly this length.
Getting an honest score
Take the test with your hands in their normal position and resist the urge to slow down for the score. A single minute is short enough that a burst of unusually careful typing can flatter your result. If you want a number that predicts real work, take three tests and use the middle one.