I plan my story by deciding what major plot events will take place in the entire story. I then make one or two occur in their own chapter. My stories tend to have a lot of travel. This makes having a new chapter occur when the characters arrive in a different location feel natural.
Depending on your story a change could be going to another room; change in perspective, or the addition/removal of a major character to a situation. For example if you have a group of marines in a battle with mercenaries and the second in command of the unit is killed it may be time to start a new chapter. Generally a major change in location, perspective or circumstance is a good reason to start a new chapter.