Astronaut Frank Rubio has been on his current flight aboard the International Space Station for more than 355 days. He launched aboard Soyuz MS-22 on 21 September 2022 and will land - according to the current plan – on 27 September 2023. He'll be then the only US astronaut who has spent a year or more in space.
The longest single spaceflight up to now is still the 438-day mission by Russian Valery Poliyakov aboard the Mir space station between January 1944 and March 1995. Additionally, six other Russian cosmonauts have spent in space longer on a single mission than Rubio.
The previous US record was held by Mark T. Vande Hei, who stayed at the ISS for 355.2 days in 2021-2022.
The most experienced human in space is Russian Gennady Padalka, who spent 878 days in space on his five flights. He's now retired.
Oleg Kononenko, who holds the sixth position in a list of most experienced space fliers with his 737 days in space on four flights, will start another mission later this September as a commander Soyuz MS-23. He is assigned to a one-year-long mission with his Soyuz MS-24 crewmate, Nikolai Chub. If the mission is more than 300 days, as usual, Kononenko will become the first person to stay 1000 days in space.
Read more about Frank Rubio's record at NASA:
An interesting list of the longest space flights and other records is on Wikipedia: