As they were a roughly military organization one can draw on similar themes...we were issued our uniforms (usually 3 to 4 standard work, 1 dress and if you were in a specific area you might have gotten additional uniforms due to the nature of your workplace - i.e. medical, mechanical, flightline, etc.).
We replaced our uniforms quite infrequently (usually only after they either withered down in colour schemes from wear/tear and washing them - the uniforms became too light; nowadays is replaced with more durable wash/wear uniforms) or if you had significant weight/height changes that required a change in sizes. The only major unnanounced replacement was if damaged and could not be repaired without noticing the repair work.
Normal duration could easily last you a tour (4 years) or so without the need to replace anything.
Military changes in uniform styles were not that common but there were a few times when each general replacing the next one decided to switch something about the organizational uniforms of the service branch...whether it was replacing name tags with velcro boards, removing shoulder ranks to only velcro patch areas with ranks, to going with naval sleeve rank stripes, etc. Overall most of these styles changed only a little and the main uniform itself only changed a few times in many years (Vietnam OD Green tiger stripe, OD Green BDU, then Desert BDUs for those serving in the Middle East, then the selected digi-patterns for some services and eventually the ACU digital pattern being utilized currently). Those were not very many changes since the 1960s - 50 years worth of military service and only about 3-4 changes and not always to the entire service.