I think the rule of thumb is a year, but i think it is silly to end a relationship or worry about ending a relationship if you think he doesnt want to marry you..
If he has talked about it, then he is thinking about it.
If you are at the stage in your life where you are comfortable and stable in your life to the point where you hear the clock ticking and you are starting to wander, it has been too long
Some people would be with someone longer than that before they start to think about marriage..
It is a very personal matter, are you willing to wait for him? Or do you think in your heart he isnt serious?
Ask him yourself... This isnt 1950, bring it up, subtle ways of course!