It always becomes unavailable on EF at about T-25 when under airport control, and I think on BA between T-25 until OLCI opens at T-24.
As far as I have been able to see, the map goes down on both channels at exactly T-25h (well, it was 30 seconds late the last time I tested it). I've come to think of the time between T-25h and T-24h as the witching hour.