The upgrade TODAY is about four years. That in no way predicts upgrade opportunities for today's new-hire.
To achieve four-year upgrades in the future, the fleet size would have to remain stable or grow (unlikely & very unlikely) and attrition would have to continue at a good clip (reasonably likely).
As was mentioned some FO's will wait until they can hold a good schedule before upgrading (with luck you can upgrade, get your TPIC, and move on before that guy comes in on top of you). Some FO's will never upgrade.