Where do I see how long my gas mask can hold?

I'd like to know how much time I have on my gas mask on the surface.

My character's breathing frequency changes sometimes. Is that an indicator that I should change my gas mask?

Also, should I change my gas mask for any other reason than if it becomes damaged, like, glass broken from an attack?

Solution 1:

There is clear indication of that on Artyom's watch.

On PC, you have to press T to look at it. On Xbox 360 - Left Bumper.

Also, should I change my gas mask for any other reason than if it becomes damaged, like, glass broken from an attack?

AFAIK, new mask always comes with the filter at 100%, so you could change just for that.

Solution 2:

In the inventory, beneath the gas mask is a number. That's the number of minutes left for your filter. It's game minutes though, not real-time minutes.

Also, when your filter is getting low your mask should start to fog up, which is an indication that you will need to change it soon.