BA generally as follows:
SH LGW 6-8 years.
SH LHR 8-10 years
LH LHR 18+ years
There can be variations on occasion. SH LHR commands were available to anyone qualified with a pulse a few years ago (under two years) but have since adjusted back upwards. I believe LGW may currently be less than above. LH has been pretty constant for the past few years, varying only by a year or so. In BA, the seniority/age demographic effectively controls the LH commands (currently about 1600/4100).