The Constitution of the United States does not set any limits on the total amount of time in office that a given person may serve in the House of Representatives and/or in the Senate. The Constitution specifies the length of the terms for members of these houses, but it does not specify how many terms they may serve. The Constitution has been amended to limit the number of terms a president may serve, but no such amendment has been made for Congress.
In the 1990s, there was a major push in many states to put term limits on members of Congress. However, the Supreme Court ruled that such laws were unconstitutional in the case of U.S. Term Limits, Inc. v. Thornton.
Thus, a person may serve in Congress for as long as they keep getting reelected.