One eventful year may merit three chapters, while you might summarize five other years in one chapter. Consider Your Audience The key to writing a great biography is really found within this idea: choose facts that are both relevant and interesting to your audience.
This also brings to the importance of fact-checking.
What were some turning points in this life? The final paragraph will summarize your main points and re-assert your main claim about your subject. Start from the point of view that you need to get endorsement or at least confirmation, and work from there: whether you decide to get a biography authorised, it will still need to be defensible we have laws about that sort of thing.
However, each biography will include the basic details.
You can also browse these short biographies of Selected Figures from Recent History. Now imagine yourself as someone asking what you thought was a perfectly reasonable set of questions and wondering why you are getting short shrift.