On the page with the stations is a Installation and Service Guide: http://www.domyownpestcontrol.com/msds/Advance_termite_system_label.pdf which explains everything about the stations.
If the stations have not been changed or checked in several years, then you should replace all of the parts in the stations so they have fresh wood and fresh inspection cartridges.
Here is a link to the parts: http://www.domyownpestcontrol.com/termite-bait-systems-c-3_75.html
Scroll down the page and you will see the wooden base, cartridges, spider tools to open it, etc.
Typically you should replace all wood and inspection cartridges (TIC) once a year. There is no set time, it all depends on how damp the soil is, weather conditions, and if the stations are sitting in areas that are more prone to dampness. Termites like damp wood, but they do not like to swim and they do not eat wood that is completely moldy. Typically, you should replace all parts of the station at least once a year.
It is possible that the TIC cartridges can decompose, or crumble sort of. They should not disappear though. Usually if they are decomposing, or look mushy, this means they are exposed to too much moisture and you should consider moving the stations a little so they are not so wet, or re-digging the hole they are in to make sure water is not puddling inside the stations. As a rule, they should not be directly under drip lines of the roof, or directly where a downspout dispenses water over the top of them, and you should dig the hole so there is space a few inches underneath the station so water can flow through it and not sit in the station.
How long the TBS cartridge takes for termites to eat it all solely depends on the size of the termite colony and how aggressive they are eating it. You should check stations at least every 3 months. The cartridges are designed to at least last that long in between monitoring so the termites do not eat it all in one 3 month period.
The TBS cartridges can be used when no termite activity is present, but it is not recommended. Not only are they much more expensive, but it has been statistically proven that termites that eat the inspection cartridge first are more likely to stay and attack the bait cartridge more aggressively once you switch them out, and the more aggressive they are the better chance you have of eliminating the colony before the termites realize they are dying. The TBS cartridges will last the same as the inspection cartridges, it all depends on exposure to moisture, etc.