Barring a catastrophic event, I think 1.5 to 3 years isn't an unreasonable forecast. A friend hired April of last year just got an Ejet upgrade. He could've done it a month or two sooner had he stuck with the CRJ. Even if incoming airframes slow down, attrition will only pick up. I'm guessing 2.5 years for guys hired the second half of 2014, but it depends on fleet growth (always rumors, rarely information of substance) and hiring at the majors (one study predicted hiring ramping up significantly in 2017).

As for this Christmas debate, the guy asked a perfectly innocent question. You never know with these things. Maybe a later class date means barely missing an upgrade, then because of that barely missing out when United comes calling, but then 20 years down the line United goes bankrupt and everyone loses their retirement, meanwhile you got hired at Delta a year later and your retirement is safe and sound. You won't know if you made the right decisions until the day you retire. That said, conventional wisdom favors seniority, and flaps made some compelling points.