Like the lifetime of your car, the lifetime of your test chamber depends on how well you take care of it. And the more severe the use, the more seriously care should be taken. Thus, humidity and high performance test chambers require more dedication to care than a benchtop temperature-only chamber.
Like your car, cleanliness is the first thing you should take care of. It is smart to wipe the system down the inside of the chamber after every humidity test. Pulling the false-wall off and checking/cleaning the actual cooling coils and heaters should be done occasionally, too. Humidity systems should have their “steam generator” cleaned frequently to avoid a build-up of minerals. With many steam generators hidden out of site, this becomes particularly important to be aware of.
The humidity detection system also needs to be checked. If yours is a “wick” based system (which standard ESPEC units use), the wick should be changed after every test to ensure reliable operation. Electronic, or solid-state, humidity sensors need to be calibrated a minimum of every six months.
Speaking of calibration, the chamber controller typically should be calibrated once a year. These days this is hard to forget because many companies have quality systems like ISO keeping track of calibrations.
If you have a smaller chamber, it probably is “air cooled”. You need to clean the condenser coil regularly to ensure it doesn't clog and impede cooling, much like you should on your home's refrigerator.
Safety systems need to be tested regularly for proper operation, as well. At the very least, run a test to make sure the over-heat protection system(s) respond. An easy way to do this is to change the setting on these overheat system to below what the chamber is currently running to get a quick check on their operation.
Pay attention to other sub-systems. If your humidity chamber has a water filter, see if it is time to replace. If you have a dry-air purge system, make sure it is actually running dry by checking it's visual indicator.
Refrigeration systems should have their gauge pressures checked and compared with their previous readings. There isn't much else you can do with a refrigeration system without calling in an expert.