How many years are in a generation?

Unfortunately, the Bible does not give a direct answer to this common question. Yet it does leave us with some observations - although these have been broadly interpreted. When we study the Scriptures, it becomes clear that the length of a generation differs from one historic period to the next. We find that the generation of the earlier history of man were longer, because people lived longer and tended to marry later in life.

In one of the genealogies given in the New Testament, it says this: "So all the generation from Abraham to David are fourteen generation, from David until the captivity in Babylon are fourteen generation, and from the captivity in Babylon until the Christ are fourteen generation " (Matt. 1:17, NKJV). Each of these three groupings were summed up as 14 generations. However, each of these 3 sets of generations did not last the same number of years. In fact, the average generation in the period from Abraham to David was approximately 64 years, but the average generation from David to the Babylonian captivity and from that captivity to Christ was about 38 years. Quite a difference when counting over hundreds or thousands of years. Today, in our secular world, a generation may be as short as 18 years like the 'baby boomers' (1946-1964) and as long as the frequent 100 years - the most common time is 40 years.

Final thought, the term " generation " may have other meanings too. The 'generation' that sees Christ's return, for example, refers to all the people living at that time.
It's differs. For example: Your parents have you and you leave home at 18 (1 generation) and then you eventually get married and have children (that's another generation) and when your children have children it's yet another generation.
This is a difficult question to answer because it simply goes by one child of a parent having a child of their own and some young people get married from 14 - 18 or much later and children so you can't put numbers on this one.

If you mean, like, Gen X or Gen Y, then its about 25 years.
It is generally considered to be 20 years.