How do you know when to turn to the I Ching for advice? That's simple: Any time you like, and as often as you like. Some people turn to the I Ching for insight and clarity on particular problems -- ie, for advice about relationships, breakups, negotiations, work dynamics, office politics, family matters and overall spiritual well-being. Others like to consult the ancient oracle every morning as a kind of general meditation to start their day.
The I Ching is excellent for receiving clarity on problems or issues that logic by itself simply can't handle -- in other words, most of the important ones in life! Use it as often as you feel a need for insights or advice, or as a tool for meditation and personal reflection.