Actually, we do. In Order, when they are looking at the tapestry, it is mentioned that Regulus' entry shows a date of death "some fifteen years previously" or words to that effect. Since Harry is, I think, fifteen in Order, that would mean Regulus died around the same time as Harry's parents, when he was a year old. Interesting coincidence, isn't it?
Well, if there's one thing you can say about Jo Rowling's books is that there is almost NEVER a coincidence. Everything she puts in those books is there for a reason.