Usually about an hour before, DH.
If you check in much before that, or if there aren't many seats available, you'll have to wait in the T5 staff travel waiting area and watch the screens to see if you've been accepted or not. After that, if you're on, nip over to the room across the corridor to get your boarding card. If the flight us departing from T5 B, or C, which most long haul flights do, you'll have to walk a bit briskly, through security and then through T5A to get across to the B or C gates.