These are single continuous flights, not cumulative time: the clock starts at launch and stops at landing. All five were made on Mir or the International Space Station, and the record has stood for three decades.
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437.7 days — Valeri Polyakov, Mir, 8 January 1994 to 22 March 1995. Still the record. Polyakov was a physician, and the flight was designed partly to test whether a human can live in weightlessness for more than a year and still work on return. He returned in good health, and the flight remains the longest single stay in space.
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379.6 days — Sergey Avdeyev, Mir, 13 August 1998 to 28 August 1999. The second-longest single flight and, for years, part of the largest cumulative total ever flown: 747 days across three missions. He spent the stay running experiments while visiting crews brought supplies and replacements.
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373.8 days — Oleg Kononenko and Nikolai Chub, ISS, 15 September 2023 to 23 September 2024. The longest ISS expedition. Kononenko spent his 1,000th day in space during the mission; he now holds the all-time record for cumulative time, 1,111 days over five flights.
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370.9 days — Sergey Prokopyev, Dmitry Petelin and Francisco Rubio, ISS, 21 September 2022 to 27 September 2023. Their ride home, Soyuz MS-22, was punctured by a micrometeoroid in December 2022 and lost its coolant; the replacement MS-23 flew up empty, and the crew stayed an extra six months while it was prepared. Rubio's 371 days set the US record for a single flight.
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365.9 days — Vladimir Titov and Musa Manarov, Mir, 21 December 1987 to 21 December 1988. The first flight to last a full year, and the first to show that a year in orbit was survivable. The crew launched and landed on the same calendar date and monitored their own physiological changes throughout the flight.
A Mars mission is usually estimated at two to three years, including six to nine months each way and time on the surface. Polyakov's 437 days is still the longest single flight, and no crew has flown longer since. The medical questions such a mission raises are not answered by any flight so far.