From your description of the continous presence of the gas, I would possibly allow a check of some kind (Listen, Spot (if gas is a cload or colored)) to determine if they notice the gas before they get the first dose.
Then follow by the steps below:
1. If not holding their breath, save or be affected by the gas. If they pass this save they can choose to hold their breath and avoid further gas saves until they pass out or resume breathing. (this might give them an extra few minutes before needing to save again)
2. If gas is still present go to Step 1.
Rather simple and realistic in that if you are stuck in a room with a gas and no way out, eventually you are going to succumb to the gas. A high level fighter might take an extra long time (if they need to roll a 1 to fail), but as long as they are not immune to the said gas, eventually it will affect them.
The real question is can they escape before they succumb. If they have a chance (no matter how slim) of finding a hidden door, unlocking the door, disabling the trap, breaking out of the room, teleporting out, or some similar way of escaping they should get the chance. If the DCs happen to be too high for any of them to make and they have no way of getting out, then tough luck.