The established standard with Git is for commit messages to have a first line that is 50 characters or less and then add two newlines, and then explain the commit throughly.

This may seem crazy short, but you’ll find that forcing yourself to summarise the commit encourages you to be atomic and concise. If you can’t summarise it in 50-80 characters, you probably are trying to commit two commits as one.

A great rule of thumb from the Homebrew Formula Cookbook.