4468,
There are very few compulsory retirements for the next 5 years, and the masses do not hit 65 for about a decade hence the stagnation and long times to get off the SH fleets.
There are many many additional variables in predicting when one reaches the major seniority number to leave SH, such as expansion, take-up of part-time, tax changes, type requirements on new types etc, but as things stand a decade on SH does not look unreasonable. I got very bored once and look at how long from my seniority as junior trash and it looked to be in the 7-9 year region. And it that length of time so much can change in IAG/BA that it could become moot.