A Co2 laser has more than just carbon dioxide in it. It has a mixture of other gasses as well. The purity of these gases and mix ratios effect the life of the tube. Running at max power can embed impurities into the cathode and reduce power. As well as degrading the mirrors, especially the output coupler. Tube life is not only the time used (turned on) but the elapsed time since manufacture. Some of the gasses can leak through the seals while sitting in storage.
If you see a sudden drop in output power, resist the urge to crank it up to finish a job. Most tube failures are due to making an existing problem worse. Kinda like flooring the gas pedal in response to someone brake-checking you.
The epilog laser is a different design with an all metal RF excited tube. They can have lifespans of up to 10,000 hours. Synrad has tube designs that last up to 45,000 hours. Granted these tubes start at 15x the cost and are for bigger machines.
If we monitor optical power output as part of preventive maintenance on a regular schedule, we can gauge the lifespan and how usage effects the life. I believe that the laser committee now has a power gauge. Not sure if it is a time/temp style or electronic.
We can actually refill our onetime use tubes with a vacuum pump and bakeout oven. Airgas sells 3-4 different gas mixtures depending if a sealed vs flowing tube. I have a 35w RF tube that I have refilled several times. RF tubes are pretty easy to fill as they have a copper fill tube vs glass.