Many textbooks come out with a new edition about every one to three years. The actual rate with which publishers release new editions largely depends on changes and advances in the field of study in question.
In new editions of textbooks editors include important new information that has come into the area of study, in addition to new modern resources and study aids that can help students. Textbooks in new fields or those that tend to change significantly come out with new editions more frequently. For example, methods and theories in subjects such as math and physics change very little over time, whereas areas of psychology and sociology change notably as new studies take place.