The answer is a long time, if you arrive at a distant T2 gate, they disembark the plane, you walk up the slopey walkway and the door into the terminal is locked (as happened to me again this week).

After the walkway was backed up completely, an airport employee finally came along and said "it's not my job to open the door, it's the airline's" !?

No BA staff to be seen (it's not their terminal). Finally got let out into the depths of T2 after a 10-15 minute delay. Grrrr.