It's one of those things that's just impossible to know. Educated guesses are possible though. The main problem is we don't know how much flying we'll get in the near future, the distant future, or if there will be another 9/11. Attrition rates aren't high now, and we can assume they'll pick up some, but people will argue how much and when.

I think people being hired the second half of 2014 will see an upgrade time of 1.5-3 years. It depends if we get more planes, and there have been rumors amounting to a net gain of ~50 in the next year. But rumors are rumors.

I don't know if the 1.5 year upgrade is sustainable, but I think 2.5 years could be.