Most couples only seek help when it is too late and the problems are insurmountable. I am not sure why, but we often see marriage problems as a sign of failure and are ashamed, instead of realising that difficulties are an inevitable part of any relationship. Ask for help when you can't sort out problems after you have tried to talk it out. Knowing when to ask for help is a sign of strength, not weeakness.
A Christian couple should seek counseling whenever they feel that their marriage is not the way it should be and that they need help to overcome particular issues in their relationship. Often Christians have the false belief that they should be able to trust God to deal with all their troubles and that if they seek counseling it is somehow an indication of a lack of faith or a failure to trust God. This is so untrue and has kept many couples struggling alone for years. The truth is that God wants His people to help each other with His guidance and truth.