My guess is sooner than that, or we wouldn't have this massive disruption of airline traffic today. We know first reports are usually misleading, but I get the feeling, based on mention that the plot was known for awhile, that perhaps the Brits thought 9/11 was the target, but some development proved that a wrong assumption.
OTOH, if this was a lucky catch by British intel, it would explain pushing the panic button today, since the plan's details would be sketchy.
They probably pushed the panic button today because their surveillance caught some men shaving their bodies and going to mosque at an unusual time.