The Bible does not give a direct answer in regards to the length of a generation. We can derive some observations. Scripture shows that the length of a generation varies, depending on the historical period. Early history of man shows longer generations because people lived much longer.
The New Testament opens with the genealogy from Abraham to Jesus Christ. At the conclusion of this genealogy, Matthew 1:17 states, “So all the generations from Abraham to David are fourteen generations, from David until the captivity in Babylon are fourteen generations, and from the captivity in Babylon until the Christ are fourteen generations.” These three groupings are all considered fourteen generations. When looking at the average generation for each group, we find that the generation length differs.
Some confuse life spans with generations. The average generation is the average age of parents at the time of birth of their children. When life span increases, so does the average length of generations. To show the extent of how life span varied over time, the average life span of the pre-Flood patriarchs was 912 years. The average lifespan from Shem to Peleg (contemporary of the Tower of Babel) diminished to 484 years, and from Peleg to Abraham, it diminished to 195 years.
The average generation from Abraham to David (Matt. 1:17) was approximately 64 years, while the average generation for the other two groups was 38 years. The time span covered by each generation is not the emphasis, but rather the fact of the passing of the number of successive generations.
There are also other meanings for the term “generation.” For example, the “generation” that will see Christ return (Matt. 24:34) refers to those who will be alive at that time.