StarSan does not know how to read a calendar. its effectiveness is a function of its pH, and as such how much organic material it's come into contact with. starsan mixed with distilled water lasts "indefinitely" (real key is to check that the pH is below 3); the same starsan mixed with too much beer or krausen or whatever is rendered useless pretty quickly.
if nothing gets into the airlock in those 6 months, I'd leave it be … not because it's starsan, but because it's an airlock, and things just don't go through it. But if you want to change it out, no big deal. heck, replace the airlock with some tinfoil and a rubber band, if you want. "bugs" (figuratively) can't crawl up. If you don't have krausen actively coming up and out of the fermenter to be contaminated at the interface, and you don't have enough clearance for fruit flies or spiders or ants or whatever to get into the carboy, then you have a reasonably isolated environment.
(note that sours are a whole different story due to the desirability of a specific amount of O₂ exchange. also, some other styles have other requirements.)
