The complexity of the idea, and the amount your readers need. This is why you must know your intended audience. A general audience needs more information than specialists do. Too little information can leave them wondering.
Inexperienced writers tend to supply too little detail.
Completeness satisfies the average reader's curiosity. American Sign Language is a language unto itself. It has its own rules that determine the meaning and significance of each gesture. Experts familiar with these rules can use their hands to communicate extremely complex thoughts. This paragraph is incomplete because the first two sentences merely state the topic.