If you’re dedicated to being a diplomat and you find that the life suits you, you can become and stay a diplomat for probably as long as you’d like. If your goal is to move up in the ranks and achieve the highest status possible (Secretary of State, Secretary General of the United Nations), take each step as it comes and make the most of wherever you’re stationed.
There are people who plan to live out their entire professional careers as diplomats and after a few years decide that it’s not the life for them. It does take a certain amount of flexibility, and although you’ll probably bring your immediate family with you—spouse, partner kids, etc.—your time away from your extended family may be too much. You may spend Thanksgiving in an all-day meeting in Indonesia or the Jewish New Year on a ferry going from the north of Senegal to the south. You may discover it’s not the life for you.
Then again, this may be your dream job and you end up living in 20 different countries over a span of 25 years. It’s really up to you as to how long you want the career to last and how much you’re willing to give to it.