Serious talk about your relationship

When is the right time to have the talk about if your going to get married or see where the relationship is going and how do you do it? I tried with my current boyfriend and he just starts talking about somthing else I've been with him for 3 years and our relationship is the same as when we started? Maybe its my fault for not saying somthing sooner to him?? How do I know if this is all that he wants a girlfriend? He's 33 years old and I'm 28. You would think we would be engaged?