Bag tags have a time to live (TTL) of 72 hours. So airlines want to keep it as close to departure as possible. I think 3 hours is probably the max here in a large US station like BOS.
Source: I walked through Delta's $100m baggage expansion at ATL when they completed it, and we chatted about this as I saw bags "off to the side" which were very early for a flight and could not be routed to a gate/plane yet.
Source 2: Friends of mine who transferred in ATL from Europe (two groups) on our way to Mexico had delayed/misplaced bags; one arrived 5-6 hours before our flight; the other maybe four hours.