IMO that's almost impossible to predict. Currently the 757/767 fleet is 56(?). Supposed to be approx. 32 in 2024? Based on where the 32nd a/c is currently manned (estimate) that number will be reached by a guy hired today in approx. 2030-2032. And right now the only word we have is a vague 'the 767 fleet will be here until at least 2028'. The last airplane? Twenty airplanes?

But as the G4 fleet increases you'd expect the G3 CA seat to go more junior. Using the data above, absent changes in bidding patterns which will occur, guys could be a G4 CA before they could be a G3 CA even if the G3 airplane still exists on the property.

That's why I think it's very hard to estimate.