About twelve inches, but I don't use it as a rule ...
Oh, I see! The maths by jtr seems a good way to guess this. Incidentally, when CX announced 180 commands over a three year period (average 60 per year) a while back, they said that CX had NEVER upgraded so many so quickly. So 60 per year is probably a maximum.
Just after that announcement was made, ASL came along with about 100 direct entry captains - so the 180 quickly became 30 or so over three years. That's a BIG decrease.
Now we are looking at extending the retirment age from 55 to 60 - so add another 5 years.
And the longer it takes, the more opportunities there are for your conditions to be changed, and your contract to be shredded, again and again and again ...
Whichever way you look at it, xcheck's guess of 12 years is a pipe dream, way too optimistic. Airlines like BA and QANTAS have quicker times to command, and Emirates is lightning quick. Not this place.