They change at least slightly every year. For example, I think last this was the first time they had gas for six months, and this is the first year I can recall them doing online shopping. However, since they are trying to compete with chase freedom, it wouldn't surprise me to see discover have 5% on gas again january -march
They change quarterly (every 3 month's). Gas should be at least 2 quarters a year.
+1 Also, they include smaller subcategories at times i.e.. gas stations, theme parks and movies.