When do kids understand the concept of "patience"?

Overall, our toddler is a happy, easy-going kid who just goes with the flow.  He can usually let things go pretty easily and just move on to the next thing.  But sometimes, he really gets fixated on wanting something.  Like, we're in the car, and he really wants noodles.  Or he sees a kid playing with a cool toy and he wants it now.  I've been trying to explain to him that he has to "wait his turn," "be patient", and that we'll get to do that "later,' but he doesn't seem to grasp this yet.  So, when will he start understanding this?  And what else can I do to help him get there?