I thought there were two sent, one about a month before and one a week before. The only email I seem to have kept is one sent 5 Feb for a 12 Mar departure earlier this year, 35 days ahead, which ties in with the month before.
Having said that I'm booked to BKK on 8 Nov and I can't recall receiving an email. But then again maybe the BA computer is watching the flood water as closely as I am.