Wikipedia defines it as 30 years. A generation is varied yes, but it isn't 100 as a generation is determined by how long to til reproduction average. How many 100 year olds do you see reproducing? Also, given that Star Wars year is the same as an Earth year (Coruscant year is 365 days and is used as the galactic standard). Also, Humans are the most plentiful species in Star Wars, so it is likely what is used to define a generation.
There are three types of generations. Given it is unlikely Obi-Wan was talking about Familial, it is either Biological (the one described above) or Societal which we have no measure for in Star Wars (these are things like Boomers, Millennials, etc).
https://www.merriam-webster.com/dictionary/generation
https://dictionary.cambridge.org/us/...ish/generation