Good question. We don't know how long ago Regulus died, but it had to have been pre-Harry. However, the contents of the note make it seem like RAB must have taken the locket post-prophecy.
Dumbledore apparently didn't know RAB had taken it, so I think it's safe to rule out Snape as the accomplice, unless Snape kept it secret from both Dumbledore and Voldemort. Could he have? (The Slytherin tendency toward self-preservation, plus a new appreciation for just how powerful Snape is, suggests "maybe.")
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?