NASA missions that have gone beyond their original Prime Mission lifetime, such as TESS, go into the NASA Senior Review process every 3 years. The most recent one of these was the 2019 Senior Review which reviewed Hubble, Chandra, Fermi, NuSTAR, NICER, Swift, Newton-XMM along with TESS. This review panel looks at the whole operation of the mission, whether it's doing good science, how the operations are conducted, how much lifetime there is left in the spacecraft and the cost of continuing operations.
The full panel report is available but all 8 missions were recommended for continued funding and TESS was rated as "best of the rest", just behind Hubble and Chandra (which have an enormous science legacy). The panel report is a recommendation to NASA but they normally follow the report as best they can and as the budget permits (Congress can, and occasionally does, intervene in this process by adding or removing funding for specific missions).
As detailed in the NASA response to the panel, the 8 missions were all extended for the Fiscal Year 2020-2022 but as noted, this is contingent on NASA's Astrophysics Division receiving the funding it asked for in the FY2020 Presidential Budget Request and the Congressional budgeting process. Extensions beyond 2022 are possible but depend on the results of the 2022 Senior Review. The TESS spacecraft is in a very stable orbit which allows it to maintain the orbit with minimal use of the propellant consumables on board (which normally sets the end of the mission as was the case for Kepler/K2 for example)
The panel noted:
The expected planetary return for TESS’s extended mission is high, pushing well beyond the prime mission. Due to the longer time baseline, together with solid Kepler exoplanet demographic statistics, the team makes a strong case that the extended mission will nearly triple the detection of planets smaller than 4 Earth radii in the extended mission, compared to the prime mission, nearly triple the number of detected planets in the habitable zone, and more than triple the number of planets with periods beyond 20 days