I always try to maintain my reports to within the last 3 to 4 months, only stepping over the bounds if the user has particularly egregious post quality or if there is something critical to check out. I really don't see the point of people reporting posts older than that: notwithstanding exceptional cases, you can anticipate that those posts would be invariably buried beneath the subsequent lackadaisical replies or beneath the newer, spammier threads in the board.