How long can a Jedi hold their breath?

This Wookiepedia article (which is based on EU canon) says:

Using the Force power breath control, a Jedi could literally numb the body's instinct to shut itself down after long periods without oxygen, allowing the user to be able to go without breathing for longer periods than he or she ordinarily could by using the Force to reserve the amounts of air in the lungs. It was believed that some users of the technique could hold their breath for hours or even days with this power.

However, in regards to the scene you mention were Obi-Wan and Qui-Gon are trapped in the room with a poisonous gas, this sentence from the Wookieepedia article should also be mentioned:

This ability used independently, however, would not prevent anything entering or leaving the respiratory system of the user. In some cases, the user would require some kind of Force barrier in, or around the nose/throat region to keep poisonous gases or water out. The necessity of such a barrier would depend on the air/water pressure in the direct vicinity, and the user's ability to naturally seal their airways

It's not stated in the film script, the official novelisation or the Star Wars in 100 Scenes factbook quite how long the Jedi were trapped in the room with the gas. The implication is that the Nemoidians were watching the events on their holographic monitor until the gas became so thick as to be opaque and for long enough for a squad of droids to be dispatched. That alone would seem to imply that Qui-Gonn and Obi-Wan were trapped for at least 5-10 minutes.

The holocam in the conference room showed only a thick green cloud. Nute Gunray studied the picture carefully. The Jedi must be dead by now, he thought. He switched to a view of the hallway outside the room, where a crowd of skeletal battle droids waited. “Go in and blast what’s left of them,” he ordered. The droids readied their weapons. One of them opened the door, and the deadly gas billowed out. Nute tensed, seeing movement in the cloud, but it was only the protocol droid. The Jedi are dead, he thought with satisfaction. He reached to shut off the screen.

_{^{Star Wars: The Phantom Menace - Official Novelisation}}

In other (less canonical) references, Jedi Masters are described as being able to hold their breath for hours or even days.