How do I find out my oxygen level in Elite Dangerous?

I have not played in a few months but when I last played the only time the countdown timer appears is when your cockpit is shattered or your life support is turned off or disabled.

You don't actually have an oxygen level in Elite: Dangerous. Instead you have a timer that triggers when your canopy is destroyed and counts down to 0. The length of timer is determined by the quality of the Life Support unit you have installed.

The default length of time is 5 minutes, which can be increased to 7.5, 10, 15 or 25 minutes. When you next dock at a station with an Outfitting section, take a look at the Life Support options that are available.

It's worth considering how much air you have available when running combat missions, as you'll die if you don't make it back to a station in time. Note that you don't have to dock with the larger stations (Coriolis, Orbis or Ocellus) you just need to make it inside.

It's also worth adding that your Life Support is (by default) one of the first systems that shuts down when you run out of power. So, if you try to shoot something and find that all of a sudden you're struggling for breath, fit a bigger power supply or look at prioritising your power-draining modules.