I think that is a number/time span that is going to rapidly change.

As stated earlier, without another aircraft order and based solely on retirements, at your age you would never upgrade.

But even with sustained growth, say hiring 300 and upgrading 150 per year, an FO hired today is looking closer to 6-7 years, unless there is several consecutive years of explosive growth.

This of course does not factor in economic cycles.