I studied the "manual" that came with the lux meter. It says that these need periodic recalibrating, and that is because the photo diode sensitivity constantly drops, proportional to the product of the time the diode is illuminated and the intensity of illumination. So, that answers my question about why they always have caps over the sensor - to keep the sensor dark to slow down the deterioration.
After 11 hours I can still smell the ammonia given off by the curing silicone. This is the first time I ever cared about how long it takes, so it is probably typical. I decided to do the final cementing of the cap when the smell is gone.