It is possible that the lenses will become dirty from the outer side. Wipe them with some soft rag and be careful not to scratch the surface. Nevertheless, they must be really dirty for the signal loss to be observable as excessive packet loss.
Maybe the sillica gel will eventually become saturated from the trace humidity that leaks inside no matter how it is sealed. It will probably manifest as high packet loss on a sunny day after rainy one and it will be possible to see water drops hanging on inner surface of the lens. Then tear off the sealant around the cap, unmount the cap, remove the silicagel, put it into oven (minimum setting for gas oven, 150degC for electric oven, do not use microwave oven) for two hours, replace the bags, replace the cap and seal it again with the sealant.
Usual fault is caused by forgetting to tighten the AUI interface connector with a screwdriver. I have seen at least two cases where the device didn't work and the AUI connector was tightly inserted, however not bolted down. Bolting it down helped.
The LED has specified lifetime 100,000 hours, which is 12 years of 24/7 operation. The light output very slowly decreases over time. If you think it is shining too dimly, then replace the diode with another one.
Do not remove the spider that will inhabitate the slot between the hood and the tube of the transmitter. Neither remove his web. The spider is a part of system design. His purpose is to protect from insect that is being attracted by the transmitter beam on the output aperture and which would otherwise gradually smudge the lens surface ;-)
What to do if one comes is described in a FAQ entry.