What causes the Summon command to appear?

After playing over 40 hours of FFXV, I've been able to summon probably around 10 times. Many times it's during boss fights, but not always. Is there some criteria that has to be met for the ability to summon one of the Six to appear? Or is it just random/luck?

For clarification, I'm not asking what determines which of the Six will appear, I'm asking what influences whether or not I'll get the prompt to summon, period.

Solution 1:

George Willcox already provided the correct source to this, but didn't answer the question.

The summons become available randomly.

As explained in the Wiki:Astral, certain circumstances increase the chance for certain summons.

Titan's chance increases the more allies are KO or in critical health. Also he needs an open area to be able to appear.

Ramuh's chance increases with battle duration and does not have additional requirements.

Leviathan's chance increases when Noctis is in critical health. She needs an open area as well, but also water nearby.

Shiva's chance increases the more allies are KO or in critical health and with battle duration. She also needs space, but it is less restrictive than for Titan and Leviathan.

Bahamut is a special case and can only be summoned at a particular point in the story.