you nailed it on the edit re: the retention period.
storage space definitely. I generally push mine to 31 days just so I can always run a "Last Month" report on the 1st. I have definitely seen it longer (up to 90 or more days) but your SQL server is going to take a hit when you ask it to do more work, so there's an obvious cost associated. I would probably increment a week at a time just to be safe. But, to be honest, in a standard environment (small/medium with the right hardware) you probably wouldn't notice much difference.
Loop1 Systems: SolarWinds Training and Professional Services