I've been doing Healing Codes for 4 years now and am a Healing Codes Coach now. I do not believe you have to really believe they work to have them work. However, if you didn't have some sort of faith in them you probably wouldn't do them, and that would be the real cause of them not working.
You also don't have to spend hours and hours. In fact, I usually can't find time for more than 2-3 Healing Codes per day for myself, though I do Haeling Codes for a number of other people (I do the quick, 1-minute version called the Q Codes--here's a review of what they are and how to use them for others).