Yet another way of looking at it: Everyone joins an airline at the age of 20 and progresses through to becoming a capt but then retires at age 65. The airline has a fixed ratio of capt to total pilots .... this ratio in CX as of the last seniority list is 998/2509 (not including those not yet allocated a seniority number). Thus as everyone progresses through their career with no airline expansion taking place and a steady rate of recruitment and retirement everyone will be made a capt after 24.1 years of service. They will then remain a capt for 15.9 years.
This scenario is pessimistic in a number of its assumptions (averge join age is prob higher, leaving age is definately lower, expansion may be occuring in fits and starts, the crew ratio driven by economic needs may well fall etc ...) However, it still comes out with a better number that the 'boundary' defined by the OP.
The above model may well provide a 'better boundary', but reasonable prediction needs to take place if the aim is to forcast how long I must remain an FO. If you throw your hands up in the air and say "there are way to many unknowns" and still come out with silly figures you might as well not take part in the predictive exercise in the first place.