Five minutes is the length many employers use: long enough that a lucky streak cannot carry the score.
Typing tests for jobs often last five minutes, and some employers only accept results from a test that long. Many US government postings for typists ask for 40 words per minute on a five-minute test, with three errors or fewer.
CamelKeys counts only correct words in your speed, so the number you see is a net speed, the one employers usually mean. Your accuracy is shown next to it. Each result is checked keystroke by keystroke on the server, and checked 5-minute results have their own leaderboard. For a result to show an employer, the free typing certificate uses the same five minutes on a passage from a classic book and gives you a page anyone can check.
Build up to it. Once your one-minute speed is steady, take two or three one-minute tests in a row, then a two-minute test, then the full five minutes. Stamina comes from steady, relaxed typing: keep your shoulders loose and your eyes on the text a word or two ahead of your fingers.
On the day, read the instructions of the employer's test carefully: some count errors against you, some do not let you correct them, and some ask for a gross speed rather than a net one.
It depends on the job. 40 WPM over five minutes is the US federal line for clerical posts, and most published office requirements sit near it; data entry is usually measured in keystrokes per hour instead. Check the posting, and see what typing speed jobs ask for.
It measures the same thing, but an employer may ask you to take their own test. Use this one to practise and to know where you stand.