The answer is "five eyes fit across the average adult’s head".
When you have the eyes set up, you can measure the width of the head. It's five eyes over.
Obviously, there is a ton of variety in human appearances, and some will be more extensive, some will be smaller than this. However, the "5 eyes wide" control is a decent place to begin.
The distance between the eyes is an indistinguishable size from an eye. You should have the capacity to fit 5 eyes over the head so utilize your pencil as an estimating guide.
You place the eyes when drawing a portrait like this;
One between the two eyes, the one between your right eye and right ear and one in the middle of your left eye and left ear.
