I'm not sure there is any set time to be honest..
I would have said 3 hours before as that is fairly standard. However, a visit to BOD saw them only open about 90 minutes before.
Actually at LGW any passenger, no matter their status and including non BAEC members, can check a bag at any time for any flight (unless there's disruption in which case that will change). And we also take bags from 4pm for any flight the following day.
As for Italian outstations, well they have no rules anyway
