I believe the COPV was capable of 10k psi [1] or ~690 bar. The 400L is in addition to the 200L tank giving 600L total, which means roughly 300 kg of nitrogen. At that pressure the Z factor is around 1.5, so it’s closer to 500 kg/m³ than the 690 the 1 kg/m³ approximation gives. With the car at just under 1700 kg [2] you need about 83 kg/s to hover at 200 m/s, so you get roughly 3.5 seconds. That’s before tank mass and pressure falloff though.

[1] https://youtu.be/6D5MlPZQFOw?si=5x7x96SqdW3Z7OzJ&t=769 [2] https://youtu.be/22rvpUecLts?si=pq6EuQnDWdH9tgBf&t=264