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.