How long should couples date before marriage?

I have been in a committed relationship for over a year and do not have an interest in dating for more than around 2 years. My boyfriend has expressed that he is not interested in marriage until his daughter graduates from high school, which is 4 years away because he has a lot of child support to pay and a spouse should not have to endure that. I do not agree with his rationale. He is divorced and I have never been married. I would love to spend the rest of my life with him but I cannot see myself dating someone for 5 or more years. What do I do?