The President is limited to two terms. Several governors face the same constraint. Congress has no limits at all, and some members have held their seats for three, four, even five decades. That is no longer public service, that's a career.
Representatives should be limited to three terms: six years. Senators should be limited to two terms: twelve years. That is still a long time in public service. It is enough time to learn the job, accomplish something meaningful, and hand it off to someone new.
The argument for keeping career politicians is that "experience matters." That is true, but experience in a system that rewards staying over serving is not the kind of experience that helps anyone. What we get instead is a Congress full of people whose primary skill is getting re-elected, whose relationships with lobbyists and special interests have had twenty years to calcify, and whose incentive is preservation rather than progress.
Fresh people bring fresh ideas. A seat that turns over keeps everyone honest. Twelve years in the Senate is still twelve years.