I voted 2024. The Reagan era and the string of close elections (since 2000) both come to an end with the election of 2024 which sends a Democrat to the White House to match Democratic majorities in Congress.
There have been (imo) four great realignments. Jefferson, Lincoln, Roosevelt, and Reagan. I think the Reagan era concludes in 2024 (Trump - Pence I don't think can hold together the Reagan coalition past 2024).
Outside chance of 2020 if the Republicans really act incompetent.
If that happens, the Dems should pack the court, because it'll probably be 7-2 conservative Republican by that point.