When do I need professional help?
If your life is disrupted by a high level of emotional pain then it is time to seek help. If you have tried a variety of resources and have not found the help you need, then it is time. Of course, you do not have to wait until your pain is high and your life is disrupted. Professional help is available at any point--and so are support groups and a variety of other resources.
I used to sail as a hobby. One of my past sailing instructors said that the time to reef your sails is when you first think of it. Sails are reefed, or partially lowered, in the case of high winds to keep the boat operational. Waiting until the wind is blowing full force makes it difficult to reef the sails safely. Reefing sails when one first thinks of it is good seamanship. In some ways, it is the same with professional help. If you go for it when you first begin to wonder if you need it, you may have a much easier time of it overall.
Actually, most people would benefit from counseling at least once in their lives--whether for one session or many. Talking with friends who have had counseling can help alleviate ones fears about calling for an appointment. Some people also find it helpful to interview more than one therapist if necessary to find one they feel can help them. Don't hesitate to shop around if you feel the need.
When should one seek professional help? When life hurts too much and nothing seems to help.
If you do need professional help then wait no longer--make an apointment and keep it.