It really depends on what you want to write about, but I always write as if someone was to read it in many years and would be interested in what I did, like I also am interested in how the life of my great-grandpa was.
If I want to add a lot of details, I do it. If I only want to write a brief description of something, I do that. It really depends on what I want to communicate.
You can also go really in-depth on a specific feeling if you want to. Maybe think about how you would read your diary if you were a 100+ year younger version of yourself. Or if you write it for future-you, think what you would like to know. Etc.