From what I have read and those who tinker with their Chinese lasers, the tubes are rated by 10,000 hour units from time of production. Better quality (and thus more dollars) tend to have longer real world lifespans.
Because the internal gases can potentially break down with usage and seep past the seals, the manufacturers rate the quality of their laser tubes. Now with the optimal performance reference, that means the laser tube will start to be less powerful (dropping from the 40 or 45 watt original rating to lower measured rates). @dan has been asked how the output is measured and he stated that they have some multi-thousand dollar testing gear.
Now understand that as manufacturing of laser tubes have improved, longevity has increased. Forums from 6 to 8 years ago talk about replacing tubes after 2 or 3 years (not sure if it failed or they wanted the rated power performance back) and the replacements are still running after 5 to 6 years.
For warranty and standardized ratings, the industry will reference 10,000 to 20,000 hours shelf life (from date of manufacturer). They cannot consistently address usage because environmental operating temperatures, load ratings and duration of projects effect the laser tube. Biggest wear on the tube is it going too hot during operations.
For hobby usage, I cannot see you needing to replace the laser tube for 3 or 4 years. I am assuming that Glowforge will have some sort of discovery for when the power output gets less than the unit can perform with and give you notification. The 40 Watt laser will probably recommend only running jobs for XX number of minutes before allowing the tube to cool, while the 45 Watt one can run longer for the same projects (and faster) before needing a cool down period.
The Chinese units keep temperatures lower by external water chillers/coolers (think your car’s radiator), while the Glowforge has peltier coolers internal that rely on the vented air to dissipate the heat build up.
Just like regular oil changes, sensible driving habits and general cleaning for a car, you can easily get 100,000 plus miles and 10 to 15 years when it is treated correctly, good ventilation, moderate temperature operational environment (if it is too cold or too hot for you, probably not good for GF ) and keep it clean and don’t touch internals that need to be left alone.