Promise Rings

Has anyone received a promise ring from their boyfriend/girlfriend? How far into the relationship was it when you got it? I want more information because I'm thinking about asking my bf if we can get one. Can you even ask for one or is that totally not cool? I'm so lost as to how this works! I know what a promise ring means, generally speaking. I'm just not sure how to go about talking to him about getting one. Anything helps!! Thanks! :D