I suppose it depends on the material. Textbooks for intro-level general courses (e.g. calculus, physics, statistics, etc.) tend to get republished yearly.

Since the fundamentals of the material are unlikely to change within the span of a year, the revisions to the textbooks are mostly cosmetic in nature—updated graphic design, rearranged problem sets (to deter the used book market), etc.

Also note: I'm speaking about the US. Things could very well be different elsewhere.