How long do I wait for him to propose?

We have been together for a little over 2.5 years and from the start have talked much about getting married and wanting kids. I was in my junior year of college when we got together and now since past may have been out of school and working. We have lived together for almost 2 years now. (and there is a big age gap I am 23 and he is 34) He was in the process of starting a new business when we first got together and is still is doing that but has not made much profit from it yet. I am naturally getting anxious for him to propose- but he will not do it until he makes some money. Is this just an excuse? I have many friends that get married because they love each other and agree to work through money woes together. Considering that we already live together and help each other with money, I dont understand what difference a ring would make. Have any of you experianced this with a guy? Is he using this as an excuse to just drag his feet?